• In any problem where information you have has different units to the information you’re being asked for, you’ll need to do a

__unit conversion__.

• Chemistry calculations involve units like number of moles (units: mol), the mass of a substance (units: g), the volume of a gas, liquid or solution (units: L) and others.

• Calculations in chemistry can be solved by breaking down questions into segments:

$\circ$ An unknown quantity to be found - the answer to the question.

$\circ$ An initial quantity to be converted into the units of the unknown quantity.

$\circ$ A conversion factor(s) linking the unknown quantity and the initial quantity.

• A conversion factor is an expression as a fraction that equates one unit to another. For example:

$\frac{1\;min}{60\;s}$ and

$\frac{60\;s}{1\;min}$
• Because the

__value__ of both terms in the unit conversion are

__equal__ (60 seconds is equal to 1 minute), when

__multiplying__ by a unit conversion the value of the expression

__doesn’t change__.

• This also means you can arrange either term (seconds or mins) on the top or the bottom; arrange it so that

__your original units cancel and you convert to the new units__. This is why it is known as a conversion factor.

•

__CONVERSION FACTORS WILL CHANGE THE UNITS WITHOUT CHANGING THE VALUE!__
•

__To solve calculations using the unit conversion method__, the following steps should be done in order:

$\circ$ Identify the unknown quantity to be found – this should be written with units and put one side of an equation.

$\circ$ Identify the initial quantity the question has given you – this starts, with units, on the other side of the equation.

$\circ$ Apply the unit conversion(s) by multiplying it with the initial quantity you were given.

$\circ$ This works even if multiple unit conversions are necessary – this method also encourages you to display your working clearly so any mistakes are usually easy to spot!

• For example: If there are 6 eggs in a box, how many eggs would be in 4.5 boxes?

$\circ$ $number\;of\;eggs = 4.5\;boxes\;*\; \frac{6\;eggs}{1\;box}=27\;eggs$
• For example (part 2): If an egg costs $2 each, how much does 3 dozen eggs cost?

$\circ$ $Cost\;(\$)= 3\;dozen\;eggs\;*\;\frac{12\;eggs}{1\;dozen\;eggs}\;*\;\frac{\$2\;egg}{1\;egg}=\$72$
• This method can be used beyond chemistry to solve any problem involving a known quantity that can be converted into another unknown quantity.

In this lesson, we will learn:

• The units of measurements commonly used in chemistry

• How to use the unit conversion method and the reason it is valuable.

• Practical examples of using the unit conversion method to do calculations in chemistry.