If 10.0 grams of NaCl is dissolved in 100.0 mL of water what is the concentration in weight/volume percent? What is the molarity?

Question

If 10.0 grams of NaCl is dissolved in 100.0 mL of water what is the concentration in weight/volume percent? What is the molarity?

Answer 1

If we assume the density of water is 1.0 g / ml, then 100.0 ml of water = 100.0 g

Weight/volume percent = 10.0 g / 100.0 g + 10.0g = 10.0 g / 110.0 g = 0.0909 = 9.09% w/v

Molarity = moles NaCl / L of solution. Since we don't know the volume of the SOLUTION, we can't really calculate molarity. We know we have 100.0 ml of water, but after adding the 10.0 g of NaCl, we don't know if we still have 100.0 ml or if we have a greater volume (likely). However, if we ASSUME NO CHANGE in VOLUME, then we can calculate molarity as follows:

molar mass NaCl = 58.44 g / mol

10.0 g NaCl x 1 mol / 58.44 = 0.171 moles

100.0 ml solution (assumed) x 1 L / 1000 ml = 0.100 L of solution (assumed)

Molarity (M) = mols / L = 0.171 mols / 0.100 L = 1.71 M