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Dealing with Density Lab...

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Dealing with Density Chem 2070 TA: J. Rein September 15, 2019

Abstract: A scale and graduated cylinder were used to determine the density of a brass bead. By obtaining the mass of the beads on a gram measurement scale and then using water displacement in a graduated cylinder to measure the volume of the beads, it was found that the density of a brass bead is 8.639 g/mL. This measurement was then used to find that the mass percent of copper in the brass bead was 85.423%. I. Introduction Density is a measurement that compares the matter found in an object to the volume of that object. Through finding the density of brass, it can reveal more about the composition of the alloy. Brass is an alloy typically made primarily of two elements; copper and zinc. To make different types of brass, different proportions of copper and zinc are used, depending on what the brass is going to be used for. It is therefore useful to find these percentages in order to gain more knowledge about the brass. The purpose of this lab was to create a procedure that would determine the percent of copper in a brass bead by finding the density of the bead II. Experimental The brass beads were first weighed using a scale. The scale was zeroed with a paper cup in it, and the beads were placed in the cup. Then, a graduated cylinder was filled with water to the 10 mL line. The beads were then placed in the cylinder and the displacement in the volume of the water was measured to find the volume of the beads. With both of these values, density could be calculated using the formula ρ=m/v. Using this value, the percent

mass of copper in the bead could be found using the formula 1/ρbead = (1− %Cu by mass)(1/ρZn) + (%Cu by mass)(1/ρCu). III. Results and Discussion To determine the mass percent copper in the brass beads, a variable was used to substitute the unknown percent copper by mass. 1/(ρ bead) (ρ bead = 8.07 g/mL) was set equal to (1-x) (1/ρ Zinc) [ρ Zinc is equal to 7.14 g/mL] plus x multiplied by (1/ρ Copper) [ρ Copper is equal to 8.96 g/mL]. The calculation is shown below:

(1/ρ bead) = (1-%Cu by mass)(1/ρZn) + (%Cu by mass)(1/ρCu) (1/8.639g/mL ) = (1-x)(1/7.14g/mL) + (x)(1/8.96g/mL) X = .85423 or 85.4%

Based on the fact that the known density of copper is 8.96 g/mL and the known density of Zinc is 7.14 g/mL, it makes sense that the density of the brass bead would be a value somewhere in between those two numbers. 8.639 g/mL is indeed a value in between these two known values, so it was accurate to proceed to use that value in the calculation of percent copper by mass in the bead. The alloy brass is typically made up of anywhere from 55% to 90% copper, so the value 85.4% also made sense.

As far as ways to improve the results, more reliable data could have perhaps been obtained if there weren’t water droplets on the sides on the graduated cylinder when measuring water displacement. These could have affected the volume and therefore change the calculation for density. This more accurate value would have influenced the result of the

density equation, creating a more reliable measure of density. Additionally, a more correct density would have created a more precise equation to determine the percent of copper by mass in the bead. Despite these considerations, the data still clearly showed a percentage that made sense in comparison to the known facts about the density of both Zinc and Copper, and the percentage of Copper used to create the Brass alloy, thus still allowing for this conclusion of the experiment to be found.

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