Question

If we have a $FeS{O}_4.7H_{2}O$ sample and it contains a total of 2233 oxygen atoms, then find the number of formula units of $FeS{O}_4.7H_{2}O$ present in the sample.

• A. 210
• B. 203
• C. 103
• D. 233

Answer 1

The correct option is B 203

1 mole of $FeS{O}_4.7H_{2}O$ contains 11 moles of O-atoms.
So, 11 moles of O-atoms are present in 1 mole of $FeS{O}_4.7H_{2}O$.
Again, $6.022\times {10}^{23}$ O-atoms = 1 mole of O-atoms.
Therefore, 2233 O-atoms $=\frac{2233}{6.022\times {10}^{23}}$ moles of O-atoms.
Hence, the number of formula units of $FeS{O}_4.7H_{2}O$ present in the sample
$=\frac{2233}{6.022\times {10}^{23}}\times \frac{1}{11}\times 6.022\times {10}^{23}=203$