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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**.

1.

Sometimes athletes take drugs to enhance their performance. One such drug is ephedrine which has the formula C_{10}H_{15}NO. 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

2.

Amphetamine, another drug, has the formula C_{9}H_{13}N. The molecular mass is 135 amu. Calculate the percentage by mass of hydrogen in amphetamine.

9.6%

80%

10.4%

0.7%

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%

3.

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.

PbCl_{6 }CO_{4}

Pb_{4}Cl_{4}C_{2}O_{6}

PbClCO_{3}

Pb_{2}Cl_{2}CO_{3}

Divide the % by RAM for each individual element. Divide those answers by the smallest which will give you your final ratios

4.

Some toothpastes contain sodium fluoride. This compound has the formula NaF. Calculate the relative formula mass of this compound.

42 amu

42

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

5.

Calculate the % by mass of fluorine in sodium fluoride.

55%

45%

48%

0.45%

19/42 x 100 = 45%. You can check this by calculating the percentage by mass of sodium - this gives 55%. 55 + 45 = 100%

6.

A tube of toothpaste contains 1.5 g of sodium fluoride. Calculate the mass of sodium in this tube of toothpaste.

0.675 g

0.825

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

7.

Calcium hypochlorite tablets are added to swimming pools to kill microorganisms. The formula of calcium hypochlorite is CaCl_{2}O_{2}. 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

8.

Calculate the percentage by mass of oxygen in calcium hyopchlorite.

22%

88%

11%

78%

2 x 16 /143 x 100 = 22%

9.

Calculate the mass of oxygen in a 50g tablet of calcium hypochlorite.

39%

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

10.

Car airbags inflate when an impact causes a mixture of chemicals to react. The mixture of chemicals contains sodium azide (NaN_{3}) 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 NaN_{3} is 170 g x(35.4/100) = 60.18 g which rounds to 60.2 g

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