### Problems

10. Calculate the average atomic weight of silicon having 92.2% of Si-28 isotope of relative mass 27.98 amu, 4.7% of Si-29 isotope of relative mass 28.98 amu and 3.1% of Si-30 isotope of relative mass 29.97 amu.

### Solution

Data given: 92.20% of silicon of mass 27.98 amu

4.7% of silicon of mass 28.98 amu

3.1 % of silicon of mass 29.97 amu

Average atomic weight

= 25.80 + 1.36 + 0.93

= 28.09 amu.

Average atomic weight of silicon is 28.09 amu.11. One million silver atoms weigh 1.79 x 1016 g. Calculate the gram atomic mass of silver.

### Solution

Number of silver atoms = 1 million = 1 x 106

Mass of one million Ag atoms = 1.79 x 1016 g

= 107.8 g

Gram atomic mass of silver is equal to the mass of 6.023 x 1023 atoms of silver. So, the gram atomic mass of silver is 107.8 g

12. Chlorophyll, the green colouring matter of plants responsible for photosynthesis, contains 2.68% of magnesium by weight. Calculate the number of Mg atoms in 2.00 g of chlorophyll.

### Solution

100 g of chlorophyll contains 2.68 g of Mg

1 mole of Mg = 24 g of Mg = 6.023 x 1023 atom of Mg

24 g of Mg contains, 6.023 x 1023 atom of Mg

=1.34 x 1021 Mg atoms

13. A sample of gaseous substance weighing 0.5 g occupies a volume of 1.12 litre under NTP conditions. Calculate the molar mass of the substance.

### Solution

1 mole of any gaseous substance at NTP occupies 22.4 L.

1.12 L of gaseous substance = 0.5 g

The molar mass of the substance therefore is 10 g/mol.