# chemistry

## Colon, Sanjiya (PT00009224)Chemistry 310

Laboratory Exercise #2 Density

Date:

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)

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

Step 1: Weigh pre-1982 pennies by difference

a.

Mass of an empty 100 mL beaker in grams.

b.

Count out 100 pre-1982 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.

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:

Part 1: Pre-1982 pennies

a. Mass of beaker = ___________________

b. Mass of 100 pennies + beaker = ___________________

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

d.

Volume of 100 pennies = ___________________ (show calculation below).

Volume of 10 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?

Experiment

Method of measurement

Volume of 1 penny in mL

1

Direct measurement (geometry)

1

Displacement

2

Calculated from mass and density (part 1 d)

Part 2: Post 1983 pennies

a. Mass of beaker = ___________________

b. Mass of 100 pennies + beaker = ___________________

c.

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

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?

Metal

Density (g/mL)

Metal

Density (g/mL)

Magnesium

1.7

Nickel

8.9

Aluminum

2.7

Copper

9.0

Zinc

7.1

Silver

10.5

Tin

7.3

11.4

Iron

7.9

Gold

19.3

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?

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Experiment 2:
Density
Chem 310
Sep 07 – Sep 11, 2021

In General: Where/How to start?
Step 1: read the introduction of the lab
To understand what this lab is about
To connect the lab material with the lecture material
Step 2: read the report sheet
To have an idea of what the instructor asking you
Step 3: read the experimental procedure
This is the most important step
This procedure is a step by step instruction on how to answer the questions in the report sheet

Experiment 2: Density
It’s about chemical properties and physical properties
Density is a physical property
The difference in densities of pre-1983 and post-1983 pennies
Step 2: read the report sheet
Asked to find mass and volume of pre-1982 pennies
Asked to find mass, volume, and density of post -1983 pennies
Answer to conceptual question 1 & 2
Asked to work on density problems
Step 3: read the experimental procedure
This procedure is a step by step instruction on how to answer the questions in the report sheet

Experiment 2: Density
Part 1: Pre-1982 pennies
Look at pictures in the experimental procedure to determine mass of beaker and mass of 100 pennies and beaker.
Mass of 100 pennies = (mass of 100 pennies + beaker) – mass of beaker
Volume of 100 pennies = mass of 100 pennies / density of Cu
Since pre-1982 pennies are made mainly with Cu (over 95%)
Volume of 10 pennies can be extrapolated from volume of 100 pennies
Volume of 10 pennies = Volume of 100 pennies / 10
Volume of 1 pennies can also be extrapolated from volume of 100 pennies

Experiment 2: Density
Part 1: Pre-1982 pennies
Look at your experiment 1 to find the average volume of 1 penny using geometry and displacement method.
Remember to convert all values of volume to mL
Accuracy and Precision in the lecture slides of Chapter 2

Experiment 2: Density
Part 2: Post-1983 pennies
Look at pictures in the experimental procedure to determine mass of beaker and mass of 100 pennies and beaker.
Mass of 100 pennies = (mass of 100 pennies + beaker) – mass of beaker
Volume of 100 pennies in part 1d = Volume of 100 pre-1982 pennies
The one that you determined using density of Cu and mass of 100 pre-1982 pennies
The assumption that volume of pre-1982 and post – 1983 pennies are similar is made here
Density of 100 post -1983 pennies= mass of 100 post 1983 pennies / volume of 100 pennies

Experiment 2: Density
Conceptual question 1 and 2
Why do we need to weigh 100 pennies rather than just one?
In your own word and thinking
Can relate to the lecture note to answer this question

Experiment 2: Density
Conceptual question 1 and 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?
In your own word and thinking
Can relate to the lecture note to answer this question
Compare the values of the three methods here

Experiment 2: Density
Density Problems:
Using the main equation:

Density = mass / volume

Other derived equations:

Mass = density x volume

Volume = mass / density

Experiment 2: Density
Please submit your lab on canvas by 11:59 pm Saturday Sep. 11, 2021.
Good luck!
Email me if you have questions or zoom with me.

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