chemistry

Laboratory Exercise #2 Density

Introduction: Density (d or the Greek letter ρ - rho) is defined as mass per unit volume and is a physical property of a substance. Physical properties of a substance are those that describe the substance without causing change to its chemical composition. Color, odor, density, melting and boiling points, hardness, metallic luster, ductility, malleability and viscosity are all physical properties.

Physical properties are not affected by quantity. For example, both one drop of water or the water of an entire swimming pool boils at 100°C at sea level. And because density is a physical property, one gram of gold has the same density as 100 g of gold. The density of solids is typically expressed as g/cm3 and the density of liquids is typically expressed as g/mL , although these units are sometimes used interchangeably since 1 cm3 = 1 mL.

To better grasp the concept of density consider the following: If the volume of two different substances, call one substance A and one substance B, are equal, but A is heavier than B, then the density of A is more than the density of B.

Density as a Physical Property

Today we’ll use the mass of a penny to determine the volume by using the density (an intensive physical property). We’ll then use the volume we determine to calculate an unknown density. Density is the ratio of how much a substance masses per unit volume (e.g. g/ml, kg/m3, g/cm3). Since density is a ratio, it’s an intensive property; the amount of substance does not affect the density so we can use density to relate mass and volume.

density = mass / volume or d = m/v

Pennies minted in pre-1982 are >95% copper with some zinc mixed into the alloy.

Pennies minted after 1982 are 97.5% zinc with a thin coating of copper to give the characteristic ‘copper’ color. Copper has the density of 8.96 g/ml, while zinc has the density of 7.14 g/ml, so pennies minted before 1982 are significantly heavier (if you have a pile of change at home and can pull out some 1982 or older pennies you can feel the difference by hand).

Reactivity as a Chemical Property

Copper does not react with H+ (acid) due to it’s positive reduction potential (an intensive chemical property). Zinc does react with H+ to form H2 gas due to its negative reduction potential. We can see the differences by watching pennies in HCl (aq), the zinc-cored penny will react, but the solid copper penny will not.

Watch a video of two different types of penny in HCl (aq)

https://www.youtube.com/watch?v=gIRFiR4vSPU

Weigh by Difference

We will be using the principle of weigh by difference to avoid systematic errors in scale

calibration. Weigh by difference takes the mass of the container alone, then the mass of the

container with the substance. The difference is the mass of the substance. This avoids any issue

of miscalibration of the scale since both values will be off by the same miscalibration so the

difference will be accurate. This method is also often used to avoid spilling materials or contamination of our scale by using a closed vial as the container.

Objective: The objective of this laboratory exercise is to introduce the concept of density.

Procedure:

Part 1: Pre-1982 pennies

c. Mass of pennies in beaker in grams.

Step 3: Use the density given above (in the introduction) to calculate the volume of 100 pennies. Extrapolate the volume of 10 pennies, and 1 penny. Compare these to the volume calculated in lab 1.

Part 2: 1983 and newer pennies

Step 1: Weigh 1983 and newer pennies by difference

a. Mass of an empty beaker 100 ml in g.

b. Count out 100 post -1983 pennies.

c. Mass of pennies in beaker in grams.

Step 2: Calculate the mass of the pennies by difference.

Step 3: Use the density given above (in the introduction) to calculate the volume of 100 pennies. Extrapolate the volume of 10 pennies, and 1 penny. Compare these to the volume calculated in lab 1.

Chem 310

Experiment #2: Density

Report sheet

Name:

Experimental data and calculations:

a. Mass of beaker = ___________________

b. Mass of 100 pennies + beaker = ___________________

c. Mass of 100 pennies = ___________________ (show calculation below).

Volume of 1 penny = (show calculation below).

e. Compare the volume of 1 penny from this experiment (part 1) to the ones that calculated in lab 1.

(You need to convert units of volumes in experiment 1 into mL) in term of precision. If density of a pre-1982 penny is 0.35 mL, how are your measurements compared to the known value in term of accuracy?

Part 2: Post 1983 pennies

a. Mass of beaker = ___________________

b. Mass of 100 pennies + beaker = ___________________

d. Using volume of 100 pennies in part 1d, and mass of 100 pennies, calculate the density of 100 post 1983 pennies.

Volume of 100 pennies in part 1d =

Density of 100 post 1983 pennies = (show calculation below).

Question 1: Why do we need to weigh 100 pennies rather than just one?

Question 2: After comparing the volume of a penny among three different techniques of experiment 1 and 2. In your opinion which method is the most precise and why?

Density Problems:

1.

A. Calculate the density of an irregular shaped metal object that has a mass of 321 g and a volume of

45.2 cm3.

B. Based on the information provided in the table below, what is the identity of the metal?

2. What is the volume of 4.00 g of air if the density of air is 1.19 g/L?

3. Calculate the mass of a quart (946 mL) of mercury. The density of mercury is 13.6 g/mL.

4. A student performed the experiment but varied the volumes used when weighing out the liquids. For the 0% salt solution, the student used 13.2 mL, for the 5% solution the student used 12.9 mL and so on. Assuming everything else was performed precisely and accurately, will the differing volumes affect the outcome of the experiment? Explain your answer.

5. What did you learn from this experiment?