Chem – Proportions and Ratios


Proportions or ratios are an important part of math in chemistry. They are normally used when comparing two different elements, compounds, or amounts. Don’t let the two words confuse you. They are actually the same thing. A proportion is a ratio and and ratio is a proportion. I know some math fanatics out there may argue that last statement with me but this a science website so I don’t care. Most importantly, what is a proportion or ratio? Quite simply, a proportion or ratio is just a type of division.


For example, the ratio of apples to oranges can be written in either of the two ways below:








Notice in both cases the thing (apples) that was asked about first in the ratio goes on top (the numerator) and the thing (oranges) that was asked about second goes on bottom (the denominator). It will be like this for all proportions or ratios unless specifically stated otherwise.


Let us demonstrate another example. If I have 40 apples total in baskets, and each basket carries 10 apples, what is the proportion of a single basket of apples to the total apples?


10 apples = 1 = 0.25
40 apples 4


For most proportion or ratio questions, the preferred representation of the answer from my example should be ¼ or 1 to 4 or 1 per 4. However, if I punch the solution into a calculator, I would get 0.25, which is the same answer but sometimes can be difficult to interpret. Again, you state the numerator first and then the denominator second. Now that you are familiar with these examples, try some practice problems on your own.


PRACTICE PROBLEMS: Solve the proportions or ratios below.


If there are 7 boys and 21 girls in a class, what is the ratio of girls to boys?

Answer: 3/1 or…..3 to 1 or……3 girls to 1 boy


From the information in the question above, what is the ratio of boys to total number of people in the class?

Answer: ¼ or……1 to 4 or…….1 boy to 4 people


If you have 80 squirrels for every 5 acres, what is the ratio of squirrels to acres?

Answer: 16/1 or……16 to 1 or……..16 squirrels for every acre


If you have 108 ounces of soda and 9 cans, what is the proportion of soda to cans?

Answer: 12/1 or……12 to 1 or………12 ounces per can




s per can

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