What is Molarity in Chemistry?
Main Learning Goal and Core Concepts: The goal of this lesson is to learn about the factors that influence molarity and how to calculate it.
Vocabulary
Molarity
Molarity is a measure of concentration of a solution. It is defined as the number of moles of solute divided by the volume of the solvent in liters. Molarity varies with temperature and pressure.
Solution
A solution is a type of homogeneous mixture made up of two or more substances. At minimum, it is composed of a substance, the solute, dissolved in another substance, the solvent.
Solute
A solute is the material that is dissolved in a solution. The solute can be a solid, liquid, or gas depending on the solution. It is typically the smaller amount in a solution.
Solvent
A solvent is the material that dissolves the solute in a solution. It is usually a liquid, but can also be a solid or a gas. The quantity of solute that can be dissolved varies with temperature.
Lesson Brief
Before molarity, there are solutions:
A solution is composed of a solvent and one or more solutes mixed together.
- The solvent is the dissolving agent
- The solute is the substance(s) dissolved
Now, what is molarity?
Molarity is a measure of concentration of a solution (not the only measure). It is defined as the number of moles of solute divide by the volume of solution in liters.
What are factors that influence molarity?
There are gas laws developed towards the end of the 18th century named by several scientists that predict changes in gases based on changing volume, pressure, and absolute temperature.
- Boyle’s Law which connects the pressure and volume of a gas
- Charle’s Law which connects the volume of a gas and absolute temperature in Kelvin
- Gay-Lussac’s Law which provides a relationship between the pressure of a gas exerted on its container and the absolute temperature
When combined together, the Combined Gas Law shown below reveals that volume is inversely proportional to pressure and directly proportional to absolute temperature.
Looking at the molarity equation shown below, molarity is inversely proportional to volume.
- When pressure goes up, volume goes down and molarity goes up. Thus, we can conclude molarity is directly proportional to pressure.
- When absolute temperature in Kelvin goes up, volume goes up and molarity goes down. Thus, we can conclude molarity is inversely proportional to volume.
Directions: Answer the following questions and select the best answer.
1. Molarity of a solution can be defined as the:
A. atomic mass of an element
B. moles of solution per liter of solute
C. moles of solute per liter of solution
D. mass of solvent per liter of solution
2. The molarity of an aqueous solution of CaCl is defined as the:
A. moles of CaCl per milliliter of solution
B. grams of CaCl per liter of water
C. grams of CaCl per milliliter of solution
D. moles of CaCl per liter of solution
3. To calculate the Molarity of a solution when the solute is given in grams and the volume of the solution is given in milliliters, you must first:
A. Convert grams to moles, but leave the volume of solution in milliliters
B. Convert volume of solution in milliliters to liters, but leave grams to moles
C. Convert grams to moles, and convert volume of solution in milliliters to liters
D. None of the above
4. What is the molarity of a solution containing 5 moles of solute in 250 milliliters of solution?
A. 20 M
B. 15 M
C. 0.104 M
D. 1.25 M
5. 25.0 milliliters of an aqueous solution of hydrochloric acid, HCl (molec. wt = 36.5) contains 11.0 grams of HCl and 18.7 grams of H2O. Calculate the Molarity of the HCl solution.
A. 0.301 M
B. 25.0 M
C. 12.0 M
D. 16.1 M
E. 10.1 M
Directions: Answer the following questions and select the best answer.
1. Molarity of a solution can be defined as the:
A. atomic mass of an element
B. moles of solution per liter of solute
C. moles of solute per liter of solution
D. mass of solvent per liter of solution
Explanation: This is the definition of molarity.
2. The molarity of an aqueous solution of CaCl is defined as the:
A. moles of CaCl per milliliter of solution
B. grams of CaCl per liter of water
C. grams of CaCl per milliliter of solution
D. moles of CaCl per liter of solution
Explanation: This is the definition of molarity applied to a substance.
3. To calculate the Molarity of a solution when the solute is given in grams and the volume of the solution is given in milliliters, you must first:
A. Convert grams to moles, but leave the volume of solution in milliliters
B. Convert volume of solution in milliliters to liters, but leave grams to moles
C. Convert grams to moles, and convert volume of solution in milliliters to liters
D. None of the above
Explanation: To calculate molarity, you must solve for the 2 components in the molarity equation: moles of the substance and liters of solution.
4. What is the molarity of a solution containing 5 moles of solute in 250 milliliters of solution?
A. 20 M
B. 15 M
C. 0.104 M
D. 1.25 M
Explanation:
Conversion of mL to L:
You must first convert the given 250 mL to L:
250 mL * (1 L / 1000 mL) = 0.250 mL
Calculation of Molarity:
Molarity = moles / liters of solution = 5 moles / 0.250 L = 20 M
5. 25.0 milliliters of an aqueous solution of hydrochloric acid, HCl (molec. wt = 36.5) contains 11.0 grams of HCl and 18.7 grams of H2O. Calculate the Molarity of the HCl solution.
A. 0.301 M
B. 25.0 M
C. 12.0 M
D. 16.1 M
E. 10.1 M
Explanation: To solve this question, you need to calculate the 2 variables from the molarity equation, molarity of solute in moles and volume of solution in liters.
Volume of solution = 25 mL
Mass of solute (HCl) = 11 g
Calculation of moles of HCl:
Moles of HCl = mass of HCl / molar mass of HCl = 11g / 36.5g = 0.301 moles HCl
Calculation of liters of solution:
Given 25 mL of solution, you need to convert mL to L. There is 1000 mL for every 1 L:
25 mL * (1 L / 1000 mL) = 0.025 L
Calculation of HCl Molarity:
Molarity = moles / liters of solution = 0.301 moles / 0.025 L = 12.04 M = 12.0 M
Note: You do not use the 18.7 g of water in calculations because that is the solvent, not the volume of solution required in the molarity formula.
Notes:
For more information on gas laws, take a look our additional information on gas laws linked here.