Question

Potassium 40 has percentage abundance of 0.0012%. How many atoms of potassium 40 are present in 225ml of milk containing 1.65 mg K/ml?

Answer 1

The total mass of potassium present in the milk sample is:

$225mL\times1.65mg/mL=371.25mg=0.37125g$

Total moles of K:

$n=\frac{0.37125g}{39.098g/mol}=9.4953\cdot10^{-3}mol$ , out of which moles of K-40 are equal to

$9.4953\cdot10^{-3}mol\times\frac{0.0012\%}{100\%}=1.1394\cdot10^{-7}mol$

Number of atoms:

$1.1394\cdot10^{-7}mol\times6.022\cdot10^{23}=6.86\cdot10^{16}$

Answer: $6.86\cdot10^{16}$ atoms