0.5 molar solution NaOH contains:

Explanation

The atomic mass of Na is 23g/mol, the atomic mass of O is 16g/mol, and the atomic mass of H is 1g/mol so, by adding all three atomic masses, we can obtain the mass number of NaOH, which is 23+16+1=40g/mol. For a solution to be 1 molar, 1 mole of a substance has to be dissolved in 1 dm3 of the solvent. Since we have to find out the mass of NaOH needed to make a 0.5 molar solution, we will dissolve 0.5 moles of NaOH in 1 dm3 of the solvent. 0.5 mol of NaOH corresponds to 20g of NaOH which can be worked out using the proportion method that if 1 mol of NaOH contains 40g of NaOH (as calculated above) then 0.5 mol of NaOH will contain 20g of NaOH. If the proportion method seems difficult, the formula relating mass with moles can be used which is: n=m/M; where M is molar mass, n is the number of moles, and m is the mass of NaOH for n moles. (0.5) = m/(40) m=(40)(0.5)=20g.