Question

numerical on mass balance equation

Answer 1

 Calculate the mass of an object from its density and volume


Density is the amount of matter, or mass, per unit volume. This example problemshows how to calculate the mass of an object from a known density and volume.

Simple Example (Metric Units)

As an example of a simple problem, find the mass of a piece of metal that has a volume of 1.25 m3 and a density of 3.2 kg/m3.

First, you should notice both the volume and the density use the volume of cubic meters. That makes the calculation easy.

If the two units were not the same, you'd need to convert one or the other of them so that they would be in agreement.

Next, rearrange the formula for density to solve for mass.

Density = Mass ÷ Volume

Multiply both sides of the equation by volume to get:

Density x Volume = Mass

or

Mass = Density x Volume

Now, plug in the numbers to solve the problem:

Mass = 3.2 kg/m3 x 1.25 m3

If you see the units won't cancel out, then you know you did something wrong! If that happens, rearrange the terms until the problem works. In this example, cubic meters cancels out, leaving kilograms, which is a mass unit.

Mass = 4 kg

Simple Example (English Units)

Find the mass of a blob of water with a volume of 3 gallons. It seems easy enough, right? Most people memorize the density of water as 1... but that's in grams per cubic centimeters! Fortunately, it's easy to look up the density of water in any units.

Density of Water = 8.34 lb/gal

So, the problem becomes:

Mass = 8.34 lb/gal x 3 gal

Mass = 25 lb

Problem

The density of gold is 19.3 grams per cubic centimeter. What is the mass of a bar of gold in kilograms that measures 6 inches x 4 inches x 2 inches?

Solution

Density is equal to the mass divided by the volume.

D = m/V

where
D = density
m = mass
V = volume

We have the density and enough information to find the volume in the problem.

All that remains is to find the mass. Multiply both sides of this equation by the volume, V and get:

m = DV

Now we need to find the volume of the gold bar. The density we have been given is in grams per cubic centimeter but the bar is measured in inches. First we must convert the inch measurements to centimeters.

Use the conversion factor of 1 inch = 2.54 centimeters.

6 inches = 6 inches x 2.54 cm/1 inch = 15.24 cm.
4 inches = 4 inches x 2.54 cm/1 inch = 10.16 cm.
2 inches = 2 inches x 2.54 cm/1 inch = 5.08 cm.

Multiply all three of these numbers together to get the volume of the gold bar.

V = 15.24 cm x 10.16 cm x 5.08 cm
V = 786.58 cm3

Place this into the formula above:

m = DV
m = 19.3 g/cm3 x 786.58 cm3
m = 14833.59 grams

The answer we want is the mass of the gold bar in kilograms. There are 1000 grams in 1 kilogram, so:

mass in kg = mass in g x 1 kg/1000 g
mass in kg = 14833.59 g x 1 kg/1000 g
mass in kg = 14.83 kg.

Answer

The mass of the gold bar in kilograms measuring 6 inches x 4 inches x 2 inches is 14.83 kilograms.

Summary of Density Formulas

Remember, you can arrange one formula to solve for mass, density, or volume. Here are the three equations to use:

Mass = Density x VolumeDensity = Mass ÷ VolumeVolume = Mass ÷ Density