Lab 2 Pre-lab Quiz - Density – A Characteristic Property (Background Reading Provided)

Each week you will complete a pre-lab exercise worth 4 points and in the form of a Canvas quiz. The rest of your lab work will be completed at the Hayden-McNeil lab simulation website. A good way to prepare for this quiz is to read the background information for the lab to be performed. You can find the links to the lab posted in the lab schedule and all of these pre-lab quizzes by clicking the "quiz" tab in the column to the left..

Please complete the following prelab quiz before Sunday, September 6th at 11:59pm. You have 30 minutes for each attempt and three attempts to get a perfect score.

Since everyone is having trouble accessing the simulated labs, I am placing the introductory reading here!

Density 

Density is defined as mass per unit volume of a substance and is a characteristic (intensive) property.  As such, the density of a substance can be used to identify that substance. 

All matter has mass and volume, which means it takes up space. The relationship between these values defines a fundamental characteristic of matter known as density. Density is a characteristic property that can be used to identify elements or compounds. Density is defined as mass per unit volume of a substance and is computed using the equation below.

d= m/V

where d is the density of a substance, m is its mass, and V is its volume.

The standard metric unit for density is grams per milliliter (g/mL). For solids, sometimes the cubic centimeter is used instead, and the unit of density is then grams per cubic centimeter (g/cm³). Numerically the two volume units are equivalent, because 1 cm³ is equal to 1 mL.

As mentioned above, the density of a substance is a characteristic property that can be used to identify matter by type. For example, the density of copper is 8.96 g/mL. Therefore, if you calculate the density of an unknown metal to be 8.96 g/mL, the metal is most likely copper.

Measuring Mass

Much of the practice of chemistry involves taking measurements. It is important to take measurements accurately and record information in an organized fashion so that it can be used at some point in the future.

The mass of a substance can be measured using a balance. There are many types of balances, but the one most commonly used is the electronic balance. An electronic balance gives fast results on a digital display. An example of an electronic balance is found in Figure 13.01.

Figure 13.01 Electronic Balance
(Thanandorn Sinphet/Shutterstock)

Measuring the Volume of a Liquid

The volume of a liquid is usually measured using volumetric equipment, such as a graduated cylinder. Due to the adhesion forces between a liquid and the walls of the cylinder, the surface of a liquid will curve and form a meniscus. The meniscus can be either concave (curving upward) or convex (curving downward). To minimize error, measure the volume of a liquid with the center of the meniscus at eye level. For a concave meniscus, measure the volume by reading the volume mark at the bottom of the meniscus as shown in Figure 13.02A. For a convex meniscus, measure the volume at the top of the meniscus as shown in Figure 13.02B.

Figure 13.02 Measuring the Volume of a Liquid with a Concave or Convex Meniscus.
(Macmillan Learning)

The scale divisions of a graduated cylinder depend on the size of the cylinder. For example, a 50 mL cylinder is usually divided into 1 mL increments (subdivisions), whereas a 10 mL cylinder may be divided into 0.1 mL increments. The volume is read one digit beyond the smallest scale division. To achieve this, estimate the position of the meniscus between two subdivisions. Therefore, for a 50 mL cylinder with 1 mL increments, read the volume to the nearest 0.1 mL increment. For a 10 mL cylinder with 0.1 mL increments, read the volume to the nearest 0.01 mL increment.

Measuring the Volume of a Solid

The volume of a solid is measured either by taking length measurements or using water displacement. Water displacement is most valuable for measuring the volume of the solids that are in lumps, powders, and other irregular shapes.

When a solid object is submerged into water, it will displace, or push up, some of the water. The volume of the object is the difference between the initial and final water volume as shown below.

V_solid=V₂−V₁

where V_solid is the volume of the solid, V₂ is the volume of water after the addition of the solid, and V₁ is the initial volume of water.