124 exam-style questions on Structure 1: Models of the particulate nature of matter with answers and explanations. Six real samples below — the full set is free with an account.
The successive ionisation energies (kJ/mol) of an element are: 738, 1451, 7733, 10540. Which group does the element belong to?
Answer: Group 2
There is a large jump between the 2nd (1451) and 3rd (7733) ionisation energies. This means two electrons are removed easily before reaching a stable inner (noble-gas) shell, so the element has 2 valence electrons and is in Group 2 (this is magnesium).
A compound contains 40.0% C, 6.7% H and 53.3% O by mass. (Ar: C = 12.0, H = 1.0, O = 16.0.) What is its empirical formula?
Answer: CH2O
Divide each percentage by its Ar: C = 40.0/12.0 = 3.33, H = 6.7/1.0 = 6.7, O = 53.3/16.0 = 3.33. Divide by the smallest (3.33): C = 1, H = 2, O = 1. The empirical formula is CH2O.
At STP (0 degrees C, 100 kPa), the molar volume of an ideal gas is 22.7 dm3/mol. What volume does 0.50 mol of oxygen gas occupy at STP?
Answer: 11.35 dm3
At STP the volume of any ideal gas = moles x molar volume = 0.50 x 22.7 = 11.35 dm3. The identity of the gas does not matter for an ideal gas.
A fixed amount of gas at constant temperature has volume 2.0 dm3 at 100 kPa. What is its volume when the pressure is increased to 250 kPa?
Answer: 0.80 dm3
By Boyle's law, P1V1 = P2V2 at constant T. V2 = (P1V1)/P2 = (100 x 2.0)/250 = 200/250 = 0.80 dm3.
In the mass spectrum of a compound, what does the peak at the highest m/z value (the molecular ion peak, M+) tell you?
Answer: The relative molecular mass of the compound
The molecular ion peak (M+) corresponds to the whole molecule that has lost one electron. Its m/z value equals the relative molecular mass of the compound. Fragment peaks at lower m/z arise when the molecular ion breaks apart, helping to deduce structure.
How many neutrons are in an atom of potassium-39 (Z = 19)?
Answer: 20
Neutrons = mass number − atomic number = 39 − 19 = 20.
124 Structure 1: Models of the particulate nature of matter questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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