WASSCE SHS
The mole is the amount of substance that contains 6.02 × 10²³ particles. Mass divided by molar mass gives the amount.
Molar mass is the mass of one mole, in grams. For carbon dioxide, carbon is 12 and each oxygen is 16, so the molar mass is 44 g. Amount in moles is mass divided by molar mass. 11 g of carbon dioxide is 11/44, which is 0.25 mol. The number of molecules is that amount times 6.02 × 10²³. Keep the unit. A bare 0.25 is not an amount of substance until you say mol.
An empirical formula is the simplest whole number ratio of atoms. Convert a mass of each element to moles by dividing by the relative atomic mass, then divide every result by the smallest number of moles. If a result is not a whole number, multiply all of them by the same small integer until they are. A molecular formula is a multiple of the empirical formula. You need the relative molecular mass to find that multiple. Without it, stop at the empirical formula and say so.
Show mass divided by molar mass. The arithmetic mark is separate from the method mark. Relative atomic masses will be given or are the familiar whole numbers: C = 12, H = 1, O = 16. Do not invent a molar mass. For an empirical formula, the order of elements in the answer should follow the question. CH₂ is not H₂C in the way most schemes print it, though a reversed order of the same ratio is usually accepted if you have said what you are doing.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
2 marks
Calculate the amount, in moles, in 11 g of carbon dioxide. Relative atomic masses: C = 12, O = 16. (2 marks)
Molar mass = 12 + 16 + 16 = 44 g. Amount = 11/44 = 0.25 mol.
3 marks
A compound contains 0.24 g of carbon and 0.04 g of hydrogen only. Find the empirical formula. C = 12, H = 1. (3 marks)
Moles of C = 0.24/12 = 0.02. Moles of H = 0.04/1 = 0.04. Ratio = 0.02 : 0.04 = 1 : 2. Empirical formula CH₂.
2 marks
The relative molecular mass of the compound in the previous question is 28. Find the molecular formula. (2 marks)
The empirical formula mass of CH₂ is 14. 28/14 = 2. Molecular formula C₂H₄.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Chemistry freeDivide the mass in grams by the molar mass. For carbon dioxide the molar mass is 44 g, so 11 g is 0.25 mol. Multiply by 6.02 × 10²³ if the question asks for the number of molecules.
The simplest whole number ratio of atoms in a compound. Find the moles of each element, divide by the smallest, and write the formula. The molecular formula may be a whole number multiple of that ratio. You need the relative molecular mass to decide. All WASSCE Chemistry past questions