Molarity of a solution is defined as the number of moles of solute in 1 L of solution. here we are given 4 mol of NaOH to be dissolved in 2 L of water the number of moles of NaOH in 2 L of water - 4 mol therefore number of NaOH moles in 1 L of water - 4/2 = 2 mol of NaOH there are then 2 mol of NaOH in 1 L
Half life means that 50% of the sample is gone at 3.3 years. This means that an additional 3.3 years (total 6.6 years) will reduce the sample a further 50% from the point at 3.3 years. In numbers, this means 50% of 50% (0.50*0.50), which is 25%.
