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Quantitative Chemistry
Calcium hypochlorite tablets are added to swimming pools to kill microorganisms.

Quantitative Chemistry

This Chemistry quiz is called 'Quantitative Chemistry' and it has been written by teachers to help you if you are studying the subject at high school. Playing educational quizzes is a user-friendly way to learn if you are in the 9th or 10th grade - aged 14 to 16.

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Quantitative chemistry is all about calculations using the mole as a unit. In math, you expect to be doing calculations but in chemistry it can come as a bit of a shock to the system! Many students panic at the mention of carrying out chemical calculations but, with a little understanding and knowledge of certain ideas and a few techniques, they can become a lot less worrying. Calculations are a good way of picking up marks in exams. Even when you don't get the answer right, you will always get credit for your working out - so always show how you arrived at your answer.

Sometimes athletes take drugs to enhance their performance. One such drug is ephedrine which has the formula C10H15NO. What is the molecular mass of ephedrine?
55 amu
165 amu
40 amu
164 amu
Total carbon = 10 x 12 = 120. Total hydrogen = 15 x 1 = 15. Total nitrogen = 14 x 1 = 14. Total oxygen = 16 x 1 = 16. Add together = 165 amu
Amphetamine, another drug, has the formula C9H13N. The molecular mass is 135 amu. Calculate the percentage by mass of hydrogen in amphetamine.
Remember, be methodical. There are 13 hydrogen atoms in the molecule so they contribute 13 x 1 =13 amu to the total. To work this out as a percentage:13 /135 = 0.096 x 100 = 9.6%
A substance used as a cosmetic by the Ancient Egyptians has been analyzed. It was found to contain 76% lead, 13% chlorine, 2.2% carbon and 8.8% oxygen. Calculate the empirical formula of this compound.
PbCl6 CO4
Divide the % by RAM for each individual element. Divide those answers by the smallest which will give you your final ratios
Some toothpastes contain sodium fluoride. This compound has the formula NaF. Calculate the relative formula mass of this compound.
42 amu
30 amu
20 amu
The value of option 2 is also numerically correct, but it is very important to include the units in all your answers
Calculate the % by mass of fluorine in sodium fluoride.
19/42 x 100 = 45%. You can check this by calculating the percentage by mass of sodium - this gives 55%. 55 + 45 = 100%
A tube of toothpaste contains 1.5 g of sodium fluoride. Calculate the mass of sodium in this tube of toothpaste.
0.675 g
0.72 g
0.825 g
Again you MUST remember your units. The quick way to this answer is to use the answer to the previous question. 55% of NaF is sodium. 55% of anything is 0.55 so 1.5 x 0.55 gives you the correct answer
Calcium hypochlorite tablets are added to swimming pools to kill microorganisms. The formula of calcium hypochlorite is CaCl2O2. Calculate the relative formula mass of calcium hypochlorite.
143 amu
91.5 amu
45 amu
70 amu
40 + 2 x 35.5 + 2 x 16 = 143 amu
Calculate the percentage by mass of oxygen in calcium hyopchlorite.
2 x 16 /143 x 100 = 22%
Calculate the mass of oxygen in a 50g tablet of calcium hypochlorite.
5.5 g
11 g
44 g
From the previous question - 22% of calcium hypochlorite is oxygen, so 0.22 x 50 gives the correct answer
Car airbags inflate when an impact causes a mixture of chemicals to react. The mixture of chemicals contains sodium azide (NaN3) which undergoes thermal decomposition to produce sodium and nitrogen. Calculate the mass of sodium that would be produced when 170 g of sodium azide decomposes.
109.8 g
60.2 g
105.6 g
58.4 g
Remember - be methodical and first figure out what calculations would be helpful. RMM of sodium azide is 65 amu therefore by mass, there is 35.4% of Na present. The mass of Na produced from 170 g NaN3 is 170 g x(35.4/100) = 60.18 g which rounds to 60.2 g
Author:  Kate Gardiner

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