$40 g$ of calcium was extracted from $56 g$ of calcium oxide
(Atomic mass of $C a = 40, O = 16$ )
i) What mass of oxygen is there in $56 g$ of calcium oxide?
ii) How many moles of oxygen atoms are there in this?
iii) How many moles of calcium atoms are there in $40 g$ of calcium?
iv) What mass of calcium will be obtained from $1000 g$ of calcium oxide?

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Solution

(i) Mass of calcium oxide= Mass of calcium $+$ Mass of oxygen
$∴$ Mass of oxygen= $56 - 40 = 16 g$
(ii) Atomic mass of $O = 16 g$
$∴$ Moles of $O = \frac{16}{16} = 1$
(iii) Moles of $C a = \frac{40}{A t o m i c m a s s o f C a} = \frac{40}{40} = 1$
(iv) $40 g$ of $C a$ is obtained from $56 g$ of $C a O$ .
Let $x$ $g$ of $C a$ be obtained from $1000 g$ of $C a O$
$∴ x = \frac{40 \times 1000}{56} = 714.28 g$

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(i) Mass of calcium oxide= Mass of calcium + Mass of oxygen∴ Mass of oxygen= 56−40=16g(ii) Atomic mass of O=16g∴ Moles of O=1616=1(iii) Moles of Ca=40AtomicmassofCa=4040=1(iv) 40g of Ca is obtained from 56g of CaO.Let x g of Ca be obtained from 1000g of CaO∴x=40×100056=714.28g