PS 02 2020 - Problem set PDF

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Problem Set 02 – Measurement, Calculation Chem 105 1. (a) Define precision and accuracy (1 sentence each). Precision: relates to the agreement among repeated measurements Accuracy: reflects how close the measured value is to the true value (b) To test the precision and accuracy of a weight scale in the laboratory, you take a small metal block that is known to be exactly 1.000 g and weigh it three times on the scale. Three measurements are reported as follows: 0.843 g, 0.842 g, and 0.843 g. Is this scale precise, accurate, both, or neither? Explain your choice in 1 sentence.

The scale is precise because the different weights around close in range of value, but is not accurate to the exact scale of 1.000g 2. If the real weight of an object is 100 grams, state whether the following measurements are precise, accurate, both, or neither, and explain why. a. 112 g, 118 g, 106 g Neither, they are not close to 100 and the numbers are not in a prox. range b. 99.7 g, 100.1 g, 100.2 g Both, numbers close in range, accurate to being 100g c. 110 g, 111 g, 110.5 g Precise, numbers are close to another, but are not accurate, far from 100g d. 105 g, 93 g, 102 g Neither, not in same range or close to 100g e. 105 ± 2 g Neither, not in an accurate range to being 100g f. 100.5 ± 0.2 g Accurate because it is very close to 100g with a small above/below range 3. How many sig figs are in the following number? 0.0509200 There are 6 4. Perform the following calculations for measured numbers. Give your answers in scientific notation with the correct units and number of significant figures. a. 124 mg + 316 mg = 440mg, there are 3 sig fig b. 613 µL + 582 µL = 1195 µL c. 410–3 mL – 210–5 mL 0.0004ml = 4x10–3 d. 85 ng  34 ng = 28x102

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e. 6.31104 pm  3.536 105 pm = 2.23x1010 pm f. 25.34 cm  1.5510–4 cm = 16.34x104 cm 5. Perform the following calculations for measured numbers. Convert your answers to the SI unit for that quantity (m, kg, m3, kg/m3). Give your answers with the correct number of significant figures. a. 77.3 g + 41.7 g =119g = 0.119 b. 123.4 Gm – 68.93 Gm = 192.3gm c. 21.7 cm  9300 cm2 20.2x104cm d. (2.76103)(7.4102) nL 2.0x106nl e. 8.110–1 µm – 3.710–8 µm 0.81x10-7 µm f. 387 pg  210–2 mL 19350ml

6. Comfortable room temperature is about 72°F. Body temperature is about 98°F. Water freezes at 32°F and boils at 212°F. The lowest temperature ever recorded at sea level (by a human) was −128.6 °F at the Soviet Vostok Station in Antarctica, on July 21, 1983. Convert these temperatures to °C and K. 72°F C= 5/9(72-32)= 22C K=C+273= 295K 98°F C=5/9(98-32)= 37C 37+273= 310K 32°F C=5/9(32-32)=0C 0+273=273K 212°F C=5/9/(212-32)= 100C 100+273=373K −128.6 °F C=5/9(-128-32)= -89C -89+273=184K 7. The diameter of a US dime is 17.9 mm, and the diameter of a silver atom is 2.88 Å. How many silver atoms could be arranged side-by-side across the diameter of a dime? (10 Å = 1 nm).

10A= 1nm 1mm= 1x106 nm (1000000) 6 6 28.8nmx1x10 =28.8x10 28.8x10 6 mm x 17.9mm= 51.6x107 silver atoms to span across a dime Did work in class: answer= 6.22x107 A 8. Which of the following represents the largest amount of matter? a. 3.0  10–1 Mg =0.3mg b. 4.5  102 mg =450mg c. 1.0  107 µg =10000mg This is the largest amount d. 2.0  108 ng =200mg e. 2.5  1013 fg =25mg 9. Lead metal (11.3 x 109 ng/ml), tungsten (0.019 kg/ml), and liquid mercury (13.5 g/cm 3) are all very dense. If chunks of lead and tungsten are placed in liquid mercury, will they sink or float? Does it matter what size the chunks are? Why or why not?

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I first converted both lead and tungsten to grams to compare density with mercury. Lead is 11.3g/ml and tungsten is 19g/ml. comparing the densities, chuncks of lead would float but tungsten would not. Size of chunks would not impact the ability to float because the density would not change, only the volume taken up.

10. The price of gold in May was $1,228.40 per ounce (1 ounce = 28.35 g). A typical gold bar such as those shown here has a mass of 12.4 kg. a. How much would one of these bars of gold cost?

1kg=35.274ounce. 12.4x35.274=473 ounces 473x1228.40=$581,033.2 Did this in class: worked out problem to be $537,000 b. The density of gold is 19.3 g/cm3. Suppose a certain gold wedding ring displaced 0.55 mL of liquid when dropped in a glass of sparkling cider. How much did this wedding ring cost? (assuming you paid the market price of gold) Density= Mass/volume.

19.3g=Mass/.55ml = mass=volumexdensity

0.55ml x 19.3g = Mass mass=10.6g 0.374 x 1228.40 = $459

1 gram = 0.0353oz = 10.6 x 0.0353= 0.374oz...