Chemical Formulae, Equations and Calculations Part 1

4 marks · Normal

Answer the questions about calcium hydroxide, Ca(OH)2.

(a)

State the relative atomic masses (Ar) for Ca, O, and H.

[1 mark]

(b)

Calculate the relative formula mass (Mr) of Ca(OH)2.

[2 marks]

(c)

What mass of Ca(OH)2 is present in 0.5 mol of the compound?

[1 mark]

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(a) Ca = 40, O = 16, H = 1

(b) 74

Common mistakes: miscounting atoms; multiplying instead of adding

Show mark scheme and worked solution

Marking points

B1 (a) · 1 mark · Correct Ar values: Ca = 40, O = 16, H = 1

M1 (b) · 1 mark · Correct calculation: 40 + (2 × (16 + 1))

A1 (b) · 1 mark · Correct Mr = 74 (follow-through)

A2 (c) · 1 mark · 0.5 mol × 74 g/mol = 37 g (accept answers within 0.5 g for rounding) (follow-through)

Worked solution

a) Ar(Ca) = 40, Ar(O) = 16, Ar(H) = 1

b) Ca(OH)2 has 1 Ca, 2 O, and 2 H:

c) Mass = number of moles × Mr = 0.5 × 74 = 37 g