Chemistry 11Introduction to Molarity

Lesson 1

Chemists need to make solutions that have precise concentrations

Pesticides must be in proper concentrations.

Food additives must be in correct concentrations.

A driver is legally impaired at 0.08 mg/mL blood alcohol content.

A new driver is legally impaired at 0.00 mg/mL blood alcohol content.

Chemists control the concentration of chemicals using the concepts we will develop in this unit.

The concentration of a solution tells you how much solute that is dissolved in a given amount of solvent (water).

The molarity is the concentration of a solution.

Molarity =Moles

Litre

1. 2.255 moles of NaCl is dissolved in 4.0 L of water, calculate the molarity.

Molarity = 2.255 moles4.0 L

= 0.56 moles/litre

= 0.56 M (Molar)

M means moles per 1 litre

= 0.356 M

0.250 L

56.11 g

x 1 mole5.00 g

Molarity =

2. 5.00 g KOH is dissolved in 250. mL of water, calculate the molarity.

3. How many grams are there in 205. mL of a 0.172 M solution of NaCl?

1 Lx 0.172 moles0.205 L x 58.5 g

1 mole= 2.06 g

= 2.62 L0.500 mole

x 1L 1 mole95.3 g

125 g x

4. How many litres of 0.500 M MgCl2 solution contain 125 g MgCl2?

5. How many grams of NaCl must be used to prepare a 100.0 mL of 0.250 M

solution.

= 1.46 g1 mole

x 58.5 gx 0.250 moles 1 L

0.1000 L

6. How many millilitres of a 0.200 M solution of CoCl2 will contain 25.0 g ?

= 962 mL1 L

x 1000 mL0.200 mol

x 1 L129.9 g

x 1 mole 25.0 g

7. Describe how you would prepare 100.0 mL of a 0.200 M solution of CoCl2.

Transfer and rinse to a 100 mL volumetric flask- fill to the line

Dissolve in water

Weigh 2.60 g of CoCl2

= 2.60 g1 mole

x 129.9 g1 L

x 0.200 moles 0.1000 L