Question 9.18: What mass of NaCl is needed to make 1.50 L of a 0.300 osmol ...
What mass of NaCl is needed to make 1.50 L of a 0.300 osmol solution? The molar mass of NaCl is 58.44 g/mol.
ANALYSIS Since NaCl is an ionic substance that produces 2 mol of ions (Na^+, Cl^-) when it dissociates, the osmolarity of the solution is twice the molarity. From the volume and the osmolarity we can determine the moles of NaCl needed and then perform a mole to mass converstion.
STEP 1: Identify known information. We know the volume and the osmolarity of the final NaCl solution.
V = 1.50 L
0.300 osmol = \left(\frac{0.300 mol ions}{L} \right)STEP 2: Identify answer and units. We are looking for the mass of NaCl.
Mass of NaCl = ?? g
STEP 3: Identify conversion factors. Starting with osmolarity in the form (moles NaCl/L), we can use volume to determine the number of moles of solute. We can then use molar mass for the mole to mass conversion.
\left(\frac{moles NaCl}{\cancel{L}} \right) \times (\cancel{L}) = moles NaCl \\ ( \cancel{moles NaCl} )\times \left(\frac{g NaCl}{\cancel{moles NaCl}} \right) = g NaClSTEP 4: Solve. Use the appropriate conversions, remembering that NaCl produces two ions per formula unit, to find the mass of NaCl.
\left(\frac{0.300 \cancel{mol ions}}{\cancel{L}} \right) \left(\frac{1 mol NaCl}{2 \cancel{mol ions}} \right) (1.50 \cancel{L} ) = 0.225 mol NaCl \\ (0.225 \cancel{mol NaCl})\left(\frac{58.44 g NaCl}{ \cancel{mol NaCl}} \right) = 13.1 g NaCl