Select the correct numerical value for the number of moles of S in two moles of H2SO4

Updated March 01, 2020

By Rosann Kozlowski

Reviewed by: Lana Bandoim, B.S.

A mole calculation in a solution requires using the molarity formula. The volume of the solution and the solution concentration is needed.

Molarity is the number of moles of solute per liter of solution. A solute, which can be solid, liquid or gas, is a substance that is dissolved in a solvent. The solvent is another substance that is capable of dissolving it within its intermolecular spaces. Together, the dissolved solute and the solvent make a solution.

Molarity is also considered molar concentration because it is the measure of a concentration of a solution. The formula for molarity may be expressed as:

M is molarity
Mol is the moles of the solute
L is the liters of solution

To fully understand molarity, the mole concept must be understood. A mole (often abbreviated to mol) is a unit of measurement. It is a certain amount. If a dozen bagels were purchased, the amount, if counted, would be 12 bagels.

A mole, like the word dozen, denotes a particular amount, too. However, the amount, called Avogadro’s number, is very large: 6.022 × 1023.

If a mole of bagels were purchased, they would nearly fill the interior space of the Earth. Although a mole of anything can be counted, it is usually reserved for incredibly small items, like atoms and molecules.

One mole of any element or chemical compound is always the same number. One mole of hydrogen would mean there are 6.022 × 1023 atoms of hydrogen.

A mole of sodium chloride, NaCl, is the same amount, 6.022 × 1023. Here, however, it is 6.022 ×1023 molecules. With molarity, consider the moles of solute as finding the number of molecules in solution.

The values for concentration and liters of solution must be given or calculated.

Example problem: Sugar, or sucrose, easily dissolves in water. How many moles of sucrose are in a 0.02 M solution?

In the problem, the molar concentration, M, is given: 0.02 M. The volume is assumed to be 1 L since the definition of molarity is moles of solute per liter of solution.

Use the formula from “Molarity Definitions and Formula” (above) to solve for moles:

Rearranging to solve for moles of solute:

mol = 0.02 mol/L × 1 L = 0.02 mol of sucrose, C12H22O11

There are 0.02 moles of sucrose in a 0.02 M sucrose solution.

Commonly, a question asks for grams of solute, especially if the substance must be measured in a laboratory setting. If the question asks for how many grams of sucrose must be added to make a 0.02 M solution, these additional steps may be followed:

While the counted amount of a mole of any substance is 6.022 x 1023, the molar mass of that substance will be different. For example, sodium chloride, NaCl, will have a different mass than table sugar, sucrose, C12H22O11.

Every element has a different molar mass, commonly located under the symbol on a periodic table. For example, one mole of carbon (C) has a mass of 12.01 g/mol. The molar mass of hydrogen (H) is 1.01 g/mol, and oxygen (O) is 16.00 g/mol.

For example, the molar mass of sucrose would be calculated by adding the molar masses of the individual elements: