1 Mole of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ weighs

1 Mole of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$  contains 1 mol of calcium atoms, 2 mol of hydrogen atoms and 2 mole of oxygen atoms. Therefore, the weight of 1 mole of $\mathrm{Ca}{\left(\mathrm{OH}\right)}_2$ weighs is equal to

$1\times \left(40\mathrm{g}\right)+2\times \left(1\mathrm{g}\right)+2\times \left(16\mathrm{g}\right)=74\mathrm{g}$