Chemistry solutes and solvents

Question

If Luke mixes 20 g of NaCl (formula wt. of 40g/mol) with enough water to make 500mL of solution. (1L=1000mL); what is the Molarity (M) of the solution ? (SW)

Answer 1

Molarity (M) is defined as moles of solute per liter of solution.

In this case, the solute is NaCl (table salt), however the formula weight or 40 g/mol is incorrect.(see below)

The solvent is water

Formula wt of NaCl = 23 for Na + 35 for Cl = 58 g/mol (NOT 40 g/mol). I will use the correct formula wt.

Since molarity = moles solute/liter solution, we must first find the moles of NaCl being used:

20 g NaCl x 1 mol NaCl/58 g = 0.34 moles NaCl (If you use 40 g/mol then you have 0.5 moles NaCl)

Liters of solution = 500 ml soln x 1 L/1000 ml = 0.5 liters of solution

Molarity(M) = 0.34 moles/0.5 liters = 0.68 M (if you use 40 g/mol, then you you 1 M)