MC test: The mole concept

Multiple choice test on 1.2 The mole concept

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet). You may find a calculator useful to answer some of the questions.

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which contains the greatest number of atoms?

1.0 mol of hydrogen atoms has a mass of 1.0 g and contains 6.02 x 1023 atoms of hydrogen. 1 mol of hydrogen gas, H2, has a mass of 2.0 g and contains 2 x 6.02 x 1023 atoms of hydrogen.
 

 

How many atoms are present in 16.0 g of methane?

One molecule of methane, CH4, contains five atoms so one mole of methane contains five times Avogadro's number of atoms, i.e. 3.01 x 1024.

 

What is the molar mass, M, of CuSO4.5H2O?

Use the values given in Section 6 of the data booklet to calculate molar masses to two decimal places. Relative molar masses, Mr, have no units, but the units of molar mass are g mol-1.

 

How many atoms are there in one molecule of ethanol, C2H5OH?

One molecule contains 9 atoms whereas one mole contains 9 x Avogadro's number of atoms.

What is the percentage of hydrogen by mass in benzene, C6H6?

M(C6H6) = [(6 x 12.01) + (6 x 1.01)] = 78.12 g mol-1 so percentage of hydrogen by mass = (6.06 ÷ 78.12) x 100 = 7.76 %

 

Which of the following have the same empirical formula?

I.  propene, CH3CHCH2

II. 1,2-dimethylbenzene, C6H4(CH3)2

III. cyclohexane, C6H12

The molecular formulas of propene and cyclohexane are C3H6 and C6H12 respectively, so both have the empirical formula CH2. The molecular formula of 1,2-dimethylbenzene is C8H10, so its empirical formula is C4H5.

 

What is the molecular formula of a compound that contains 8.522% carbon, 1.433% hydrogen and 90.04% iodine by mass and has a molar mass of 281.86 g mol-1?

in 100 g, amount of C = (8.522 ÷ 12.01) = 0.71 mol, amount of H = (1.433 ÷ 1.01) = 1.42 mol and amount of I = (90.04 ÷ 126.90) = 0.71 mol so the empirical formula is CH2I. The molar mass is twice the empirical mass so the molecular formula is C2H4I2.

 

Which of the following have the same empirical formula and the same molecular formula?

I.   ethyl methanoate, HCOOCH2CH3

II.  propanoic acid, CH3CH2COOH

III. methyl ethanoate, CH3COOCH3

They all have the empirical formula C3H6O2 and the same molecular formula C3H6O2.

 

Which contains the greatest mass of chloride ions?

0.2 mol of CaCl2 contains 0.4 mol of Cl− ions. Both 0.1 mol of FeCl3 and 0.3 mol of NaCl contain 0.3 mol of Cl− ions. 0.1 mol of ZnCl2 contains 0.2 mol of Cl− ions.

 

In a laboratory experiment a student found that 8.630 g of magnesium burned in air to form 13.174 g of magnesium oxide. Based on these results how should she report the empirical formula for magnesium oxide?

Amount of magnesium in 8.630 g  = 8.630 ÷ 24.31 = 0.355 mol. Mass of oxygen combining with magnesium = (13.174 - 8.630) = 4.544 g. Amount of oxygen in 4.544 g = (4.544 ÷ 16.00) = 0.284 mol. Ratio of Mg : O = 0.355 : 0.280 = 1.25 : 1, so simplest ratio = 5 : 4 and the empirical formula should be reported as Mg5O4.

 

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