A spherical balloon of 21 cm diameter is to be filled up with hydrogen at N.T.P from a cylinder containing the gas at 20 atmospheres at 27^oC. If the cylinder can hold 2.82 litres of water, calculate the number of balloons that can be filled up.

Sodium when burnt in excess of oxygen gives sodium oxide.

The metallic lustre exhibited by sodium is explained by

Give reasons of the following :
Magnesium oxide is used for the lining of steel making furnace.