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Technique: Weighing Objects This module aims to familiarize you with the following principles of weighing: 1. 2. 3. 4. 5.

How to choose between a top-loading and an analytical balance. Technical pointers for more precise weighing. Correct technique for weighing with a top-loading balance. Correct technique for weighing with an analytical balance. The advantages of and correct technique for weighing by differences.

Balances What equipment is used?

In this course the measurement of mass is achieved by use of either a top-loading and/or analytical balance.

When to use a top-loading balance?

The top-loading balance is used for preparative work when obtaining a mass to the nearest ±0.1 g or ±0.01 g is acceptable. It is often used for pre-weighing samples in order to determine the approximate mass of the sample. If asked in a procedure to weigh 0.8 g of a substance, a range of 0.7 to 0.9 g is acceptable. DON’T waste time measuring out exactly 0.8 g. Following pre-weighing, a more precise mass is determined using an analytical balance. The top loading balance has an uncertainty of ±0.05 g Click here to view the video on how to correctly use a top loading balance

When to use an analytical balance?

An analytical balance is used for analytical work, where the mass of a sample must be determined more precisely. It provides the mass of an object to the nearest ±0.0001 g. Masses determined on an analytical balance must always be reported to four decimal places. The top loading balance has an uncertainty of ±0.00005 g Click here to view the video on how to correctly use an analytical balance The most precise reading will be the first stable mass indicated by the balance. The reading is considered stable if it persists for 2 seconds.

Questions on Balances Determine which balance, top loading or analytical should be used for each of the following tasks: • Measuring the mass of a lead salt precipitate to the nearest 0.1 g. • Weighing out anhydrous solid copper sulfate for a reaction where the procedure calls for 1.5 g of this chemical. • Determining the water content of hydrated copper sulfate by weighing a sample of this salt prior to and following drying in an oven. • Weighing a sample of a solid to four significant figures.

General Notes on weighing A warm or hot object will create a convection current in the air Weigh objects at room around the balance pan. This fluctuating force reduces the air temperature. pressure on the balance pan and can make it difficult to obtain a stable reading. Weigh only dry objects. Use approximate amounts and then measure them precisely.

Moisture can corrode the balance pan. Moisture evaporating from your sample can lead to unstable mass readings. Approximate amounts should be weighed out on a top-loading balance and then weighed precisely on an analytical balance. A range of ±0.1 g from the desired mass is acceptable.

Use the same balance.

It is critical to always use the same balance especially if calculations require more than one mass measurement, as each device might be calibrated slightly differently.

Use weigh boats.

Weigh boats are containers used to prevent reagents from contacting the balance pan. They are made of polypropylene, a plastic that does not adsorb water. They are inexpensive and do not need to be handled with care. If one is torn or too dirty to be wiped clean, simply discard it.

Reading the balance

The most precise reading will be the first stable mass. The reading id=s considered stable if it persists for 2 seconds

Errors associated with analytical balances Source of Error

Remedy for Error

Air currents lifting the balance pan.

Close the sliding doors on the balance.

Wet samples becoming lighter as the moisture they contain evaporates off.

Make sure that the sample is dry before it is weighed.

Ultra-dry samples becoming heavier as they adsorb moisture from the atmosphere.

Let the sample sit at room temperature for five minutes prior to weighing it.

Weighing warm objects creates a convection current that lifts the balance pan.

Let the sample sit at room temperature for five minutes prior to weighing it.

Handling glass objects with bare fingers deposits oil on them and increases their mass.

Hold object through a strip of paper or wear latex gloves.

Breathing on a glass object condenses moisture on it and increases its mass.

Don't breathe on the object.

Weighing by difference When to do this? Why do this?

When using an analytical balance to weigh a chemical. When transferring a weighed chemical from a weigh boat to a beaker, the full amount of weighed chemical is almost never completely transferred, as some will stick to the weigh boat. The technique of weighing by differences, allows you to know the exact mass of a weighed chemical in a beaker. Click here to view the video on the correct way to weigh by differences.

Top Loading Balance In this course, a top loading balance is used first to roughly weigh the required amount of chemical for the experiment. Put clean empty weigh boat on top loading balance

Tare the top loading balance to 0.00 g

Add ~ mass (±20%) of chemical to weigh boat

Then the weigh boat containing the roughly weighed chemical is taken to the analytical balance to be weighed more precisely.

Analytical Balance Make sure ALL doors closed on analytical balance

Tare the analytical balance to 0.0000 g

Weigh the sample + weigh boat and record the value (1)

Transfer sample into a beaker. Do not remove any sample particles stuck on the emptied boat.

Weigh the emptied weigh boat and record the value. (2)

Be sure that the balance returns to the tare value of 0.0000 g when finished.

The difference between the two recorded values, (1) and (2) will give the mass of the sample successfully transferred into the beaker.

Questions about Weighing by Difference You are weighing out a sample of copper sulfate by differences. You place a weigh boat onto a toploading balance, tare the balance with the empty weigh boat, and measure out your sample to within the desired range, you complete the weighing by differences on an analytical balance and transfer a precisely measured amount of the copper sulfate into a beaker. You realize you used a dirty weigh boat which contained small amounts of copper sulfate. How does this affect your result? Do you need to repeat your measurement?

Summary This tutorial on weighing objects has presented the following topics: 1. How to choose between a top-loading and an analytical balance. 2. Technical pointers for more precise weighing. 3. Correct technique for weighing with a top-loading balance. 4. Correct technique for weighing with an analytical balance. 5. The advantages of and correct technique for weighing by differences.

Technique: Crucible Filtration This section will go through the principles of crucible filtration with the following topics: 1. 2. 3. 4.

Overview of filtration. How to consider accuracy when designing a filtration procedure. The advantages of filtration with a sintered glass crucible. Proper experimental technique for filtration with a sintered glass crucible.

What is filtration?

The separation of a solid from a liquid by pouring the mixture through a membrane with small holes (pores). The relatively large solid particles cannot pass through the membrane, whereas the liquid, comprised of tiny molecules is able to pass through.

What types of membranes are commonly used?

The two most common types are filter paper and sintered glass crucibles.

What types of filtration are there?

Quantitative filtration is used when trying to collect every single particle of solid material produced in a chemical reaction. Preparative filtrations are performed when this requirement does not need to be met.

What types of preparative filtrations exist?

Gravity filtration - using a glass funnel and filter paper Vacuum filtration – using a Buchner funnel and filter paper

What types of quantitative filtrations exist?

Vacuum filtration - using a sintered glass crucible

QUANTATATIVE FILTRATION Quantitative Analysis deals with determination of the amount or percentage of one or more constituents of a sample If quantitative analysis is performed on a solid that is filtered from a mixture with a liquid, two requirements must be fulfilled: 1. the filtration needs to be quantitative, trapping all of the solid particles on the membrane. 2. after separation but prior to analytical weighing, it is necessary to thoroughly dry the solid, usually by exposing it to elevated temperature in a laboratory oven. The high temperature causes the trace liquid particles in the solid to evaporate, leaving behind a highly pure sample of solid that is ready to be weighed accurately A sintered glass crucible looks like a funnel that has a thick flat membrane of opaque glass placed across the bottom end. This disk is called a frit, and contains millions of microscopic pores that allow liquid to flow through but quantitatively trap solid particles. A vacuum filtration is necessary when using sintered glass crucibles since the membrane is substantially thicker than a filter paper, and its pores are very small. Watch the video on the proper technique for using a sintered glass crucible

For quantitative analysis, a sintered glass crucible is the most appropriate equipment to use, rather than filter paper, because: 1. of being made of glass it can be exposed to high oven temperatures during the drying process. Its Pyrex glass construction makes it heat-stable up to 550°C. Filter paper will begin to decompose in the oven 2. all material to be filtered is contained within the glass crucible throughout the filtration and drying process. With filter paper, the filtered material would need to be transferred from the filter paper to a beaker for the drying process and invariably some of the wet filtered solid material will remain on the filter paper even with a carefully executed transfer.

Question 1: Filtration Would a membrane with large pores or one with smaller pores be more likely to necessitate a vacuum filtration? Would a more viscous or a less viscous liquid phase in a mixture be more likely to require a vacuum filtration?

Question 2: Sintered Glass In the tutorial above, you are instructed not to touch the sintered glass crucible with your bare hands after it has been weighed for the first time. If you were to disregard those instructions and leave oil behind on the crucible, would the calculated mass of your collected filtered end product be too large or too small?

Summary Technique: crucible filtration has discussed the following topics: 1. 2. 3. 4.

Overview of filtration. How to consider accuracy when designing a filtration procedure. The advantages of filtration with a sintered glass crucible. Proper experimental technique for filtration with a sintered glass crucible.

Measured and Exact Numbers: CHEM 115 ALL LABS 2020W Introductory Chemical Laboratory I

2021-03-29, 5*34 PM

Measured and Exact Numbers In chemistry, as in experimental sciences in general, numbers fall into 2 categories: measured and exact. In the chemistry laboratory measured numbers include those obtained when determining the mass of a compound or the volume of a solution. Characteristic of all measured values is the fact that there is some degree of uncertainty, or unreliability, associated with them. Exact numbers, on the other hand, are known with certainty. The following discussion is intended to serve as a guide for understanding, reporting and calculating numerical results. Measured numbers Every observed measurement made is really an approximation. ! For example, the length of the object in Figure 1 is between 1.5 and 1.6 units. Its length is reported to be approximately 1.54 units. There is uncertainty in the last digit 4; it is esti!mated.

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Figure 1 In general, on a linear scale, the human eye is capable of estimating the position of a mark situated between two of the smallest divisions to the nearest one-fifth (1/5 th= 0.2) of the smallest division. Thus in the diagram above, the smallest division is imagined to be divided into five equal sections and, as a result, the edge of the grey object is estimated to be at the second of these sections. Since each section is 0.2 of the smallest division it is equal to 0.04 units and the length of the object is reported to be 1.54 units. When a measured quantity is given as a digital readout which does not fluctuate, uncertainty still exists in the measurement. For example, when a digital balance indicates the mass of an object as 2 6 grams there is uncertainty in the 6 When recording a measurement it is the last digit that https://canvas.ubc.ca/courses/67638/pages/measured-and-exact-numbers

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Measured and Exact Numbers: CHEM 115 ALL LABS 2020W Introductory Chemical Laboratory I

2021-03-29, 5*34 PM

as 2.6 grams, there is uncertainty in the 6. When recording a measurement, it is the last digit that represents some degree of uncertainty and, in the case of our example, this means that the object was weighed to the nearest tenth (0.1) of a gram and that its exact mass is between 2.5 g and 2.7 g. Exact numbers As noted above, exact numbers are known with certainty. Exact numbers are numbers that are not measured experimentally and include defined conversion factors such as 1 mL = 1/1000 L (there are exactly 1000 mL in 1 litre, by definition), 1 kg = 1000 g or 1 minute = 60 seconds. Counting numbers, such as the number of samples tested in an experiment, are also exact numbers. Exact numbers are exact, not estimated, and therefore have no uncertainty associated with them. They are said to contain an infinite number of significant figures and thus ! in a calculation have no effect on the final result.

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PRECISION and ACCURACY: CHEM 115 ALL LABS 2020W Introductory Chemical Laboratory I

2021-03-29, 5*34 PM

PRECISION and ACCURACY Essential to quantitative scientific measurement is an understanding of how reliable a measurement is. Precision and accuracy are two terms related to a measurement’s reliability. Only a brief consideration of their meaning will be given here. Precision refers to the consistency of measurements with each other; in other words, precision describes the reproducibility of a result. If a quantity is measured several times and the values agree closely with each other, the measurement is precise. If the values differ widely, the measurement is not very precise. Poor precision is often a result of poor technical skills. Accuracy describes how close a measurement is to the true, or known, value. It is possible to have a series of reproducible measurements which are incorrect – precision is good and accuracy is poor. It is also possible to have a series of very poorly reproduced measurements which average around the correct value – precision is poor and accuracy is good. Ideally a procedure is both precise and accurate. In terms of the equipment used in the laboratory, more precise equipment provides measurements containing a greater number of significant figures. If this laboratory equipment is both properly calibrated and correctly used, the result will also be more accurate. Example of Precision and Accuracy In a titration 3 trails were performed with the amount of titrant being used in each trial 25.35mL, 25.33mL and 25.36mL. This result shows that the results were precise. However, the actual amount that should have been used was 26.47mL. This makes these results PRECISE but NOT ACCURATE. Note: In Experiment 1 you will only be using accuracy as you perform the experiment only once. However, in Experiment 2 you will be using accuracy and precision. High precision but low accuracy - results are reproducible but not close to the know value

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PRECISION and ACCURACY: CHEM 115 ALL LABS 2020W Introductory Chemical Laboratory I

2021-03-29, 5*34 PM

High accuracy but low precision - results are close to the know value but not reproducible

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ERRORS: CHEM 115 ALL LABS 2020W Introductory Chemical Laboratory I

2021-03-29, 5*34 PM

ERRORS Two types of errors, random and systematic, contribute to a measurement’s uncertainty. Systematic errors stem from flaws in the equipment or design of the experiment. If this is the case and an experiment is conducted with the same equipment in the same way, the error is reproducible and the results are precise but inaccurate. Theoretically, systematic errors can be determined and corrected. Poor accuracy is associated with systematic errors. Random errors are those errors over which the experimenter has little or no control. Random errors are always present and they have an equal chance of being either positive or negative. Measuring the mass of the same object on the same analytical balance three times and obtaining 3 different masses that differ by 0.0002 g is due to the internal balance mechanism and is an example of a random error. Random errors can never be completely eliminated. Random errors are associated with poor precision. Please note that in your lab reports error discussions should only refer to systematic and random errors. Mistakes, or human errors, such as spilling solutions, using the wrong solution, forgetting to take a reading, using dirty glassware, incorrectly using the balance, or making calculation errors, all of which can be corrected by doing the step over, are not valid experimental errors. These mistakes should be included in the observations portion of your Design Form and every attempt should be made to correct the mistake; time permitting, you may be required to start over.

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Technique: Standard Solutions INTRODUCTION A standard solution is a solution with a concentration known to at least 4 significant figures.

PREPARATION OF A STANDARD SOLUTION (using a Volumetric Flask) Prior to making a standard solution, you must determine its desired concentration. In In this course, standard solutions are made using a solid chemical compound, therefore once the concentration of the solution has been determined, you must calculate the mass of the compound required. The steps to make a standard solution using a volumetric flask are as follows: 1. Weigh and dissolve the solid compound. Weigh by difference (See Technique: Weighing Objects, “Weighing by Difference”) the solid compound into a clean beaker. Using a clean stirring rod, completely dissolve the solid in a small amount of deionized water, or the solvent suggested in the experiment.

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2. Clean the volumetric flask and funnel. Rinse the flask, stopper and funnel three times with deionized water. 3. Quantitatively transfer the dissolved sample into the volumetric flask. Using a funnel, pour the solution down the clean stirring rod and into the volumetric flask. Rinse the beaker, funnel and stirring rod thoroughly three times with deionized water from a water bottle so that all the rinses go into the volumetric flask. 4. Fill the volumetric flask to a few centimeters below the...