O-LevelChemistryStoichiometry5070/22 · May/June 2019 · Q57 Marks
W is a compound containing carbon, hydrogen and oxygen.
- (a)W contains 57.1% carbon and 4.8% hydrogen by mass. Calculate the empirical formula of W. .................................................................................................. empirical formula ...........................................[3]
- (b)A 0.194g sample of W reacts completely with 18.5cm³ of 0.250mol/dm³ KOH. One mole of W reacts with three moles of KOH. Calculate the relative formula mass of W. ............................................................................................. relative formula mass ..........................................[3]
- (c)Deduce the molecular formula of W.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [3] 5(a) element C H O % 57.1 4.8 38.1 moles 4.76 4.8 2.38 simplest mole ratio 2 2 1 % of oxygen (1) moles / mole ratio (1) empirical formula C2H2O (1)
- [3] 5(b) moles of KOH = 0.0185 × 0.250 OR 0.004625 (1) moles of W = moles of KOH ÷ 3 OR 0.004625 ÷ 3 OR 0.00154 (1) M₁ = (0.194/0.00154) = 125.97 / 126 (1)
- [1] 5(c) C6H6O3 (1)
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