2 Step Molarity

2 Step Molarity

Today we will be combining the knowledge we gained during the mole calculations and the molarity

Open Ended

How many moles are present in 300 grams of CaCO3 (gram formula mass = 100 g/mol)

Remember:

moles =

mass =

gfm =

moles = (mass/gfm)

Multiple Choice

What is the molarity of a 1.5 L solution containing 3 moles of CaCO3?

What is the molarity in this question?

• 3 moles

• 1.5 L

• unknown

Multiple Choice

What is the molarity of a 1.5 L solution containing 3 moles of CaCO3?

What is the liters of solution in this problem?

• 3 moles

• 1.5 L

• unknown

Multiple Choice

What is the molarity of a 1.5 L solution containing 3 moles of CaCO3?

What is the moles of solute in the problem?

• 3 moles

• 1.5 L

• unknown

Open Ended

What is the molarity of a 1.5 L solution containing 3 moles of CaCO3?

Molarity = M

moles of solution = 3

Liters of solution = 1.5

Remember:

Molarity = moles of solute/liters of solution

Why 2 steps?

• Some questions will ask for the molarity, but instead of giving you the number of moles, it will provide you with the mass and the gram formula mass.

• Step 1: Find the # of moles

  # of moles = given mass

                      gram formula mass

  
  

• Step 2: Find the molarity of the solution

  Molarity = moles of solute

                   Liters of solution

  
  

Sample Problem 1:

What is the molarity of a 2.0 L solution containing 145 grams of NaCl (gram formula mass = 58 g/mol)

Sample Problem 2:

What is the molarity of a 6 L solution containing 303 grams of KNO3 (gram formula mass = 101 g/mol)?

mass =

gfm =

moles =

Open Ended

Step 1: Find the number of moles.

mass = 303

gfm = 101

moles = mol

Remember: moles = mass/gfm

Sample Problem 2:

What is the molarity of a 6 L solution containing 303 grams of KNO3 (gram formula mass = 101 g/mol)?

moles = 3

Liters of solution =

Molarity =

Open Ended

Step 2: Find the molarity

moles = 3 mol

liters of solution = 6L

Molarity = M

Remember:

Molarity = moles of solute/Liters of solution