O-LevelChemistryStoichiometry5070/21 · May/June 2019 · Q57 Marks
Acid U is a compound containing carbon, hydrogen and oxygen.
- (a)A 6.30 g sample of U contains 1.68g of carbon and 0.14g of hydrogen. Calculate the empirical formula of U.[3]
- (b)A 0.086 g sample of U is completely neutralised by 12.7cm³ of 0.150mol/dm³ KOH. One mole of U reacts with two moles of KOH. Calculate the relative formula mass of U.[3]
- (c)What is the molecular formula of U?[1]
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [3] mass of oxygen / 4.48 (1)
- [3] moles / mole ratio (1)
- [3] empirical formula CHO₂ (1)
- [3] moles of KOH = 0.0127 x 0.150 OR 0.001905 (1)
- [3] mole of U = 0.5 × moles of KOH OR 0.001905 × 0.5 OR 0.0009525 (1)
- [3] M_r = (0.086 / 0.0009525) =90.3/90 (1)
- [1] C₂H₂O₄ (1)
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