Unit conversions in chemistry

Unit conversions in chemistry

Lessons

• 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: 1min60s\frac{1\;min}{60\;s} and 60s1min\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 numberofeggs=4.5boxes6eggs1box=27eggsnumber\;of\;eggs = 4.5\;boxes\;*\; \frac{6\;eggs}{1\;box}=27\;eggs

• For example (part 2): If an egg costs $2 each, how much does 3 dozen eggs cost?
\circ Cost($)=3dozeneggs12eggs1dozeneggs$2egg1egg=$72Cost\;(\$)= 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.
  • 1.
    Introduction to unit conversions
    a)
    Units and calculations in chemistry.

    b)
    What is a unit conversion?

    c)
    Unit conversion method: Walkthrough


  • 2.
    Apply the conversion factor method to simple calculations.
    Use the unit conversion method to answer the following problems.
    a)
    If a car can travel 75 kilometres in 1 hour, how far can it travel in 4.5 hours?

    b)
    An electronics store has an offer that sells two TVs for $430. How much would 8 TVs cost using this deal?

    c)
    At a market, a man traded 8 apples for 14 oranges. How many oranges can he get if he traded 22 apples at this rate?


  • 3.
    Apply the conversion factor method to chemistry-related calculations.
    Use the unit conversion method to answer the following problems.
    a)
    If a container holds 3.5 dozen oxygen atoms, how many oxygen atoms in total will 3 full containers have in them?

    b)
    If one mole of oxygen gas has a mass of 32 g, what is the mass of 3.5 moles of oxygen gas?

    c)
    If one molecule of white phosphorus has 4 atoms of phosphorus in it, how many molecules of white phosphorus would be needed to have 72 atoms of phosphorus?

    d)
    If one mole of hydrogen gas fills up 22.4 L in a gas canister, how many moles of hydrogen gas would fill up a gas canister 180 L in size?


  • 4.
    Apply the conversion factor method to chemistry-related calculations with SI units.
    Use the unit conversion method to answer the following problems.
    a)
    4.7 moles of carbon dioxide gas has a mass of 206.8 g. What is the mass of 1 mole of carbon dioxide gas?

    b)
    Gold has a density of 19.3 grams per millilitre (g/mL), what would the volume be of 62 grams of gold?

    c)
    If an acid has a concentration of 3 moles per litre, how many litres of acid would I need to have 1.8 moles of acid?