Question 7.C-L.4: Express the concentrations of each of the following solution...

a. The data may be substituted directly into Equation 7.5:

M = \frac{moles  of  solute}{liters  of  solution}                (7.5)

M = \frac{1.25  mol  solute}{2.50  L  solution} =  0.500 \frac{ mol   solute}{  L  solution}

The solution is 0.500 molar, or 0.500 M.

b. In this problem, the volume of solution must be converted to liters before the data are substituted into Equation 7.5:

(225   \cancel{mL}   solution) (\frac{ 1  L}{1000   \cancel{mL}}) = 0.225  L  solution

M = \frac{0.486  mol  solute}{0.225  L  solution} =  2.16  \frac{mol  solute}{L  solution}

The solution is 2.16 molar, or 2.16 M.

c. In this problem, the volume of solution must be converted to liters and the mass of solute must be converted to moles before the data can be substituted into Equation 7.5:

(100   \cancel{mL}   solution) (\frac{ 1  L}{1000  \cancel{mL}}) = 0.100  L  solution

(2.60   \cancel{g}   \cancel{NaCl})(\frac{1  mol  NaCl}{58.44  \cancel{g}   \cancel{NaCl}}) = 0.0445  mol   NaCl

In the last calculation, the factor \frac{1  mol  NaCl}{58.44  f   NaCl} comes from the calculated formula weight of 58.44 u for NaCl. The data are now substituted into Equation 7.5:

M = \frac{0.0445  mol  NaCl}{0.100  L  solution} = 0.445 \frac{mol  NaCl}{L  solution}

The solution is 0.445 molar, or 0.445 M.