What is the mass fraction of salt in a solution obtained by mixing 18 g of sodium nitrate and 140 ml of water?

Water has a density of 1 gram / cm3. Therefore, the mass of water will be 140 grams.
Let’s find the mass of the solution. To do this, add the masses of water and salt.
M solution = M water + M salt = 18 + 140 = 158 grams;
To find the mass fraction of salt, it is necessary to divide the mass of salt by the mass of the solution.
With salt = M salt / M solution = 18/158 = 0.114 = 11.4%;