Molarity Practice

Question

6c. Complete the table:

___________________________________________________________________

| Solute | Mass of solute | Moles of solute | Volume of solution | Molarity of solution |

| MgSO₄ | 0.538 g | ____________ | 29.0 mL | _________________|

| NaOH | ____________ | ____________ | 105.0 mL | 1.45 M |

| CH₃OH | 14.0 g | _____________ | _____________ | 0.520 M |

Complete the row of CH₃OH.

Enter your answers numerically separated by a comma.

Moles of solute, volume of solute = ____________________ mol, mL

Answer 1

For the CH3OH row you are given the mass (14.0 g) and the molarity (0.520 M), and you need the moles and the volume. Step 1 - molar mass of CH3OH. Count the hydrogens carefully: there are FOUR of them, three on the carbon and one on the oxygen. Writing CH3OH as if it had three H is the single most common mistake on this row. C: 1 x 12.011 = 12.011 H: 4 x 1.008 = 4.032 O: 1 x 15.999 = 15.999 Total = 32.042 g/mol Step 2 - moles of solute. n = 14.0 g divided by 32.042 g/mol = 0.4369 mol, which rounds to 0.437 mol. Step 3 - volume. Rearrange the definition of molarity, M = n / V in liters, to get V = n / M. V = 0.4369 mol divided by 0.520 mol/L = 0.8402 L Convert: 0.8402 L x 1000 mL/L = 840. mL Answer: 0.437 mol, 840. mL Three things worth noticing here. 1. The blank is labeled volume of solute, but the column heading above it says Volume of solution, and the column heading is the correct one. Molarity is always moles of solute per liter of SOLUTION, never per liter of solvent and never per liter of the pure solute. 14.0 g of pure methanol occupies only about 18 mL on its own; the 840 mL is what you have after diluting it up. If a molarity problem ever has you reaching for the solute's density to get a volume, you have taken a wrong turn. 2. Significant figures. Both given values carry three (14.0 and 0.520), so both answers get three. Write the volume as 840. mL with the decimal point after the zero, or as 8.40 x 10^2 mL. Plain 840 mL reads as two sig figs and can cost you the point. 3. Watch the units on the answer line. It asks for mol and mL, but molarity is defined per liter, so the division hands you liters and you still have to multiply by 1000. Dropping that factor is the other classic loss of points on this problem. The same two moves finish the rest of the table. For the MgSO4 row, convert mass to moles first, then divide by the volume in liters. For the NaOH row, run it backwards: multiply molarity by liters to get moles, then multiply moles by the molar mass to get grams.