What is the molarity of an H2SO4 solution of 0.25 liter of the solution contains 0.75 mol of H2SO4

Molarity is a unit of concentration, measuring the number of moles of a solute per liter of solution. The strategy for solving molarity problems is fairly simple. This outlines a straightforward method to calculate the molarity of a solution.

The key to calculating molarity is to remember the units of molarity (M): moles per liter. Find the molarity by calculating the number of moles of the solute dissolved in liters of a solution.

• Calculate the molarity of a solution prepared by dissolving 23.7 grams of KMnO4 into enough water to make 750 mL of solution.

This example has neither the moles nor liters needed to find molarity, so you must find the number of moles of the solute first.

To convert grams to moles, the molar mass of the solute is needed, which can be found on certain periodic tables.​

• Molar mass of K = 39.1 g
• Molar mass of Mn = 54.9 g
• Molar mass of O = 16.0 g
• Molar mass of KMnO4 = 39.1 g + 54.9 g + (16.0 g x 4)
• Molar mass of KMnO4 = 158.0 g

Use this number to convert grams to moles.

• moles of KMnO4 = 23.7 g KMnO4 x (1 mol KMnO4/158 grams KMnO4)
• moles of KMnO4 = 0.15 moles KMnO4

Now the liters of solution is needed. Keep in mind, this is the total volume of the solution, not the volume of solvent used to dissolve the solute. This example is prepared with "enough water" to make 750 mL of solution.

Convert 750 mL to liters.

• Liters of solution = mL of solution x (1 L/1000 mL)
• Liters of solution = 750 mL x (1 L/1000 mL)
• Liters of solution = 0.75 L

This is enough to calculate the molarity.

• Molarity = moles solute/Liter solution
• Molarity = 0.15 moles of KMnO4/0.75 L of solution
• Molarity = 0.20 M

The molarity of this solution is 0.20 M (moles per liter).

To calculate molarity:

• Find the number of moles of solute dissolved in solution,
• Find the volume of solution in liters, and
• Divide moles solute by liters solution.

Make certain to use the correct number of significant figures when reporting your answer. One easy way to track the number of significant digits is to write all your numbers in scientific notation.

Definitions

Solute-the substance being dissolved

Solvent-the substance doing the dissolving (the larger amount)

Solution- a homogeneous mixture of the solute and the solvent

Solution= solvent + solute

Aqueous (aq)= water solution

Tincture= alcohol solution

Amalgam= Mercury solution

Molarity (M)- is the molar concentration of a solution measured in moles of solute per liter of solution.

The molarity definition is based on the volume of the solution, NOT the volume of water.

Vocab. Lesson

 Incorrect= The solution is 5.0 Molarity.

Correct= The solution is 5.0 Molar.

 Example Problems

Level 1- Given moles and liters

Determine the molarity when 3.0 moles of sucrose are dissolved to make 2.0 liters of solution.

Level 2- Given Molarity and liters of solution

 Determine the number of moles of salt dissolved in 5.0 liters of a 0.50M solution of salt water.

cross multiply, X= 2.5 mols

Level 3- Given grams (instead of moles) and liters of solution

Determine the molarity when 117g of NaCl are dissolved to make 0.500 liters of solution.

   1st convert to moles, 2nd plug into the molarity equation

117g NaCl( 1mol/58.5g)= 2.00mol NaCl

Level 4-Given grams (instead of moles) and milliliters of solution (instead of liters)

Determine the molarity when 55.5g of CaCl2 are dissolved to make 250.mL of solution.

1st convert to moles, 2nd convert to liters, 3rd plug into the molarity equation

55.5g CaCl2( 1mol/111g)= 0.500mol CaCl2

250.ml( 1L/1000mL) =0.250L

Past Regents Questions-Follow link to check the answers

Jan 2003-44 What is the molarity of a solution of NaOH if 2 liters of the solution contains 4 moles of NaOH?

(1) 0.5 M     (3) 8 M
(2) 2 M       (4) 80 M

Jan. 04-41 What is the molarity of a solution containing 20 grams of NaOH in 500 milliliters of solution?

(1) 1 M (2) 2 M (3) 0.04 M (4) 0.5 M

Jan 2002-42 What is the molarity of a solution that contains 0.50 mole of NaOH in 0.50 liter of solution?

(1) 1.0 M      (3) 0.25 M
(2) 2.0 M      (4) 0.50 M

Aug. 2006-42 How many total moles of KNO3 must be dissolved in water to make 1.5 liters of a 2.0 M solution?

     (1) 0.50 mol     (2) 2.0 mol     (3) 3.0 mol     (4) 1.3 mol

Aug 2005-

41 What is the total number of moles of NaCl(s) needed to make 3.0 liters of a 2.0 M NaCl solution?(1) 1.0 mol      (3) 6.0 mol

(2) 0.70 mol    (4) 8.0 mol

June 2006-

16 Molarity is defined as the(1) moles of solute per kilogram of solvent(2) moles of solute per liter of solution(3) mass of a solution

(4) volume of a solvent

Aug 2008-

15 Which phrase describes the molarity of a solution?(1) liters of solute per mole of solution(2) liters of solution per mole of solution(3) moles of solute per liter of solution

(4) moles of solution per liter of solution

June 2009-46 Which sample of HCl(aq) contains the greatest number of moles of solute particles?
(1) 1.0 L of 2.0 M HCl(aq)
(2) 2.0 L of 2.0 M HCl(aq)
(3) 3.0 L of 0.50 M HCl(aq)
(4) 4.0 L of 0.50 M HCl(aq)

June 2007-

13 A 3.0 M HCl(aq) solution contains a total of(1) 3.0 grams of HCl per liter of water(2) 3.0 grams of HCl per mole of solution(3) 3.0 moles of HCl per liter of solution

(4) 3.0 moles of HCl per mole of water

June 2010-14 The molarity of an aqueous solution of NaCl is defined as the(1) grams of NaCl per liter of water(2) grams of NaCl per liter of solution(3) moles of NaCl per liter of water

(4) moles of NaCl per liter of solution

Jan 2008-

15 Which unit can be used to express solution concentration?(1) J/mol     (3) mol/L

(2) L/mol    (4) mol/s

Jan 04-41 What is the Molarity of a solution containing 20 grams of NaOH in 500 milliliters of solution?(1) 1 M      (3) 0.04 M

(2) 2 M      (4) 0.5 M

Jan 2010-40 What is the molarity of 1.5 liters of an aqueous solution that contains 52 grams of lithium fluoride, LiF, (gram-formula mass =26 grams/mole)?(1) 1.3 M    (3) 3.0 M

(2) 2.0 M    (4) 0.75 M

on to ppm or Molality

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