Molarity Guided Practice – Keep this Resource Sheet - Not Homework
The first concentration we will look at is the most commonly used. This is molarity. Since it starts with mol, we may guess that it deals with moles, and that is correct. Put simply, it is how many moles of a compound are present in the volume of solution in liters. It works in two directions, grams to molarity, or molarity to grams.
Molarity Step 1:Write down the formula(s) you need
For example: What is the molarity of 20 g of NaOH dissolved in 0.35 L of solution? (Molar Mass NaOH = 40 g/mol) As we are looking for molarity, and have mass, we will have two equations. We write M = moles/liters and n=m/M
Molarity Step 2:Circle what you are looking for, and derive the equation(s) for that variable.
Check the problem to see which variable you are looking for, and find it in those terms. For example: What is the molarity of 20 g of NaOH dissolved in 0.35 L of solution? We are looking for molarity so that equation is set up. We start with grams and need moles, so the other equation is set up as well.
Molarity Step 3:Plug in your knowns to determine your unknown
Put the values in for your knowns. For example: What is the molarity of 20 g of NaOH dissolved in 0.35 L of solution? 20/40 = 0.5 moles 0.5 moles/0.35 L = 1.43 M NaOH
Examples: What is the molarity of 15 g of HCl dissolved in 0.5 L of solution? (Molar Mass HCl = 36.46 g/mol)
What is the molarity of 6.0 mol of F2 dissolved in 0.25 L of solution? (Molar Mass F2 = 38 g/mol)
How many grams of KI does it take to make 0.1 L of .4 M KI? (Molar Mass KI = 166 g/mol)
How many moles of CuSO4 does it take to make 0.25 L of .25 M CuSO4? (Molar Mass CuSO4 = 159.62 g/mol)
Now you try: What is the molarity of 8.6 mol of NaOH dissolved in 0.85 L of solution? (Molar Mass NaOH = 40 g/mol)
What is the molarity of 22 g of HCl dissolved in 1.5 L of solution? (Molar Mass HCl = 36.46 g/mol)
How many grams of NaCl does it take to make 0.75 L of .37 M NaCl? (Molar Mass NaCl = 58.44 g/mol)
How many moles of KI does it take to make 1.7 L of 4.3 M KI? (Molar Mass KI = 166 g/mol)