Molarity



Summer Review Assignment #5

Molarity

2000-2001

Bob Lee

Eugene Yum

Michael Fink

David Synn

Perfected By Aditya Shah and Eric Nakamoto

Revised and made better by Jacob Nastaskin

Molarity

Molarity (moles/liter = M) is the most common concentration unit used for solutions. Molarity is defined as the number of moles of a solute dissolved in 1liter of solution.

# moles solute

molarity (M) = ---------------------

# liters solution

NOTE: Moles are derived from the equation

1 mol

moles = grams (given) * -----------------

molar mass

Molarity can also be used to calculate:

• the volume of the solution, if the number of moles of solute is given

• the number of moles of solute, if the volume of the solution is given

From the moles of solute calculated, you can also find:

• the molar mass of the solute, if the weight of the solute is given

• the grams of the solute, if the molar mass of the solute is given

EXAMPLE: A solution is prepared by dissolving 16.00 grams of NaCl in water to make 0.600 L of solution. Calculate the molarity of the solution.

First convert the grams of NaCl to moles.

The molar mass of NaCl is 58.44 grams.

1.00 mol NaCl

16.00 grams NaCl * --------------------- = 0.274 moles NaCl

58.44 g NaCl

Then solve for the molarity of the solution.

0.274 mole NaCl

M = ------------------------ = 0.457 M NaCl

0.600 L

Note: the units used for molarity (moles/L) is also used later on as the units in solubility and concentration.

Molarity is also used in finding the concentration of ions when a dissociation reaction occurs. The concentration depends on the coefficients of the reaction as it dissociates into different ions. The concentration of one particular ion that is formed can be found by multiplying the number of liters of solution to the coefficient of that ion.

EXAMPLE: HCl(l) ( H+ (aq) + Cl- (aq)

For this reaction the ratio of all the coefficients is 1 to 1. In a water solution, HCl dissociates into one mole of hydrogen ion and one chloride ion. So in one liter of solution the concentration of the hydrogen ion is 1 M. It is also 1 M for the concentration of the chloride ion.

PRACTICE: Find the concentration of the hydrogen ions, and the sulfate ions of the dissociation of 3 moles sulfuric acid (H2SO4) in 1.5 L of solution. Assume no volume change.

Answer: [H+] = 4.0 M ; [SO4-2] = 2.0 M

Assignment #5

1) Calculate the concentration of 1260 mL solution containing 76.00 grams of MgCl2 .

Answer: 0.633 M MgCl2 .

2) How many moles of K2SO4 is in a 200 mL of a 0.0760 M solution? How many grams?

Answer: a) 0.0152 mol K2SO4 b) 2.649 grams K2SO4

3) 36 grams of solid NH4Cl is dissolved with water to form a 0.007 M solution. Find the

amount of solution in liters.

Answer: 96.1 L

4) 2.00 moles of solid sulfur is dissolved in water to form a 9.00 L solution. Find the molarity.

Answer: 0.222 M S

5) How many grams of NaOH are dissolved in 200. mL of solution if its concentration is 1.50 M?

Answer: 12.0 g of NaOH

6) Calculate the molarity of a solution containing 10.0 grams of sulfuric acid in 500. ml of solution.

Answer: 0.204 M

7) To dilute 50.0 ml of 3.50 M sulfuric acid to 2.00 M sulfuric acid, what volume must it be diluted to?

Answer: 87.5 ml

8) If 25 ml of .11 M sucrose is added to 10 ml of .52 M sucrose, what would be the resulting molarity?

Answer: 0.23 M

9) If 46 g of MgBr2 is dissolved in water to form 0.50 L of solution, what is the molar concentration of Bromine ions in the solution.

Answer: 1.0 M

10) What is the molar concentration of a solution containing 37.5 g Ba(MnO4) 2 per L, and what is the molar concentration with respect to each type of ion?

Answer: 0.100 M; 0.100 Ba+2; 0.200 M MnO4 –

11) What volume of 0.123 M NaOH, in mL, contains 25.0 g NaOH?

Answer: 5.08 L

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