Why do you add the mass of water to the mass of salt dissolved in it to find the mass of the solution?

Answer 1

Nope. The volume does not change but the density of the mixture increases. No, the volume of water is straight forward to measure but salt as a solid composed of small crystals has an aparent volume wich includes many voids filled with air. Assuming you have measured properly the two components volumes adding them will provide you with what is known as ideal volume, but reality deviates from this linear additive values. Especially with electrolytes such as sodium chloride the final volume differs from ideal addition values expected.The topic has been extensively studied and explained by science, look for partial molar volumes concept and equations in basic physical chemistry textbooks. Remember that mass is conservative and that density is defined as mass per unit volume, say grams per milliliter. It is right to say that density changes ( measuring densities is the way to construct partial molar volumes graphs), since mass is conservative the only way to get density changes is by increasing or decreasing volumes. Solutions are classified as ideal (additive volumes) or non ideal (non additive volumes). Hope this solves your question.