How many (a) molecules (b) hydrogen atoms (c) oxygen atoms are there in 0.5 mol of water?

(a) 1 mol of water contains 6.022 × 10²³ molecules

     ∴ 0.5 mol of water contains 6.022 × 10²³/2 molecules

                                                  = 3.011 x 10²³ molecules

(b) 1 molecule of water contains 2 atoms of hydrogen

1 mol of water contains 2 × 6.022 × 10²³ atoms of hydrogen

   ∴ 0.5 mol of water contains (2 × 6.022 × 10²³)/2 atoms of                     hydrogen

                                               = 6.022 × 10²³ atoms of hydrogen

(c) 1 molecule of water contains 1 atom of oxygen

1 mol of water contains 6.022 x 10²³ atoms of oxygen

   ∴ 0.5 mol of water contains 6.022 × 10²³/2 atoms of oxygen

= 3.011 x 10²³  atoms of Oxygen