Molarity Example Problem

Calculate Molarity of a Sugar Solution

Sugar cubes are pre-measured blocks of sucrose. You can calculate the molarity of a solution made from dissolving sugar in water.
Sugar cubes are pre-measured blocks of sucrose. You can calculate the molarity of a solution made from dissolving sugar in water. André Saß / EyeEm / Getty Images

Molarity is a unit of concentration in chemistry that describes the number of moles of a solute per liter of solution. Here's an example of how to calculate molarity, using sugar (the solute) dissolved in water (the solvent).

Molarity Chemistry Question

A 4 g sugar cube (sucrose: C12H22O11) is dissolved in a 350 ml teacup filled with hot water. What is the molarity of the sugar solution?

First, you need to know the equation for molarity:

M = m/V
where M is molarity (mol/L)
m = number of moles of solute
V = volume of solvent (Liters)

Step 1 - Determine number of moles of sucrose in 4 g

Determine the number of moles of solute (sucrose) by finding the atomic masses of each type of atom from the periodic table. To get the grams per mole of sugar, multiply the subscript after each atom by its atomic mass. For example, you multiply the mass of hydrogen (1) by the number of hydrogen atoms (22). You may need to use more significant figures for the atomic masses for your calculations, but for this example, only 1 significant figure was given for the mass of sugar, so one significant figure for atomic mass is used.

Add together the values for each of the atoms to get the total grams per mole:

C12H22O11 = (12)(12) + (1)(22) + (16)(11)
C12H22O11 = 144 + 22+ 176
C12H22O11 = 342 g/mol


To get the number of moles in a specific mass, divide the number of grams per mole into the size of the sample:

4 g/(342 g/mol) = 0.0117 mol

Step 2 - Determine the volume of solution in liters

The key here is to remember you need the volume of solution, not just the volume of solvent. Often, the amount of solute doesn't really change the volume of the solution, so you can simply use the volume of solvent.

350 ml x (1L/1000 ml) = 0.350 L

Step 3 - Determine the molarity of the solution

M = m/V
M = 0.0117 mol /0.350 L
M = 0.033 mol/L

Answer:

The molarity of the sugar solution is 0.033 mol/L.

Tips for Success

  • Watch the number of significant figures you use from the periodic table and throughout your calculations. This can affect you final answer slightly.
  • Remember you need volume of solution, not volume of solvent. For solutions made by mixing two liquids, this can be particularly important. You can't always add together the volumes to get the final volume. For example, if you mix alcohol and water, the final volume will be less than the sum of that of alcohol and water. The concept of miscibility comes into play.
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Helmenstine, Anne Marie, Ph.D. "Molarity Example Problem." ThoughtCo, Oct. 10, 2016, thoughtco.com/molarity-example-problem-609570. Helmenstine, Anne Marie, Ph.D. (2016, October 10). Molarity Example Problem. Retrieved from https://www.thoughtco.com/molarity-example-problem-609570 Helmenstine, Anne Marie, Ph.D. "Molarity Example Problem." ThoughtCo. https://www.thoughtco.com/molarity-example-problem-609570 (accessed November 23, 2017).