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Molar Concentration is the concentration of a solute in a solution. Molar Concentration is also known as Molarity. Molar Concentration Formula is the formula which determines the amount of solute is a solution.
In everyday life, one often comes across different types of solutions. These solutions may be of three types:
- Gaseous Solutions,
- Liquid Solutions,
- Solid Solutions.
Pure substances are often rare in daily life. Everything around us is some kind of solution be it the air, the food or the water. The importance of the solutions in our life depends on their composition. For example, air with lower concentration of Oxygen is generally hard for us to breathe in.
Read more: States of matter
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KeyTerms: Solution, Solute, Solvent, Concentration, Moles, Atomic mass, Molarity, Molality, Temperature, Molar Concentration, NaOH
What is Molar Concentration?
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Molar Concentration is the number of moles of solute that can dissolve is 1 Liter of a solution. It is the most convenient way of determining the concentration of a solute in a solution. It may be defined as the total number of moles of a solute that is dissolved in a liter of a solution.
For example, 0.25 mol/L solution of NaOH means that 0.25 mol of NaoH is there is a liter of solution.
Molar Concentration or Molarity is expressed in terms of mol/L which can be abbreviated as molar or M.
Molar Concentration is often not convenient as the volume of most solutions is dependent on temperature. This may be resolved by introducing temperature correction factors.
Molar Concentration Formula
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The formula of Molar Concentration may be written as:
| M = nV |
where,
M= Molarity,
n= Number of moles of a solute,
V= Volume of solution in Liters.
With the help of this formula, the molar concentration of a substance in any kind of solution can be easily determined.
It is to be noted that in case the mass of a solute is given, convert it into terms of moles. Moles of an atom or solute can easily be found by dividing the mass by the molar mass or atomic mass.
For example, to find out the number of moles present in 25g of Lithium, we need to divide 25g of Lithium by its atomic mass which is 6.94 which is 25g6.94g = 3.62 moles.
Read more: Unit of volume
Steps to Calculate Molar Concentration
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- To determine the molar concentration of a solution, firstly find the mass of the solute which is the grams of the dissolved substance. Problems usually state the mass, although one may have to convert to grams from some other unit.
- Then, find the number of moles of a solute. For that, calculate the molar mass of the substance. Use the atomic mass of the elements and multiply the mass by the number of atoms per molecule of that element if the elements are repetitive.
For example, Acetic acid is CH3COOH. The molecule has a total of 2 carbon atoms, 2 oxygen atoms, and 4 hydrogen atoms. Multiply the atomic mass of carbon by 2, oxygen by 2 and hydrogen by 4 and then add the results to get the total molar mass in grams per mole like: (12.01×2)+(16.00×2)+(1.008×4)
where, 12.01, 16.00 and 1.008 are the atomic mass of Carbon, Oxygen and Hydrogen respectively.
That gives us 60.05 grams per mole.
- Now, to calculate the moles of the solute, divide the mass in grams by grams per mole.
For example, we have 10g of acetic acid. Then by dividing 10g by 60.05 g per mole will give 0.1665 moles of solute.
- Finally, substitute the values of the number of moles of solute and the volume of solution in the molar concentration formula and simplify.
For example, the acetic acid here is completely dissolved in 1.25 L of water. Therefore, Molarity = 0.1665 mole/ 1.25 L which will be 0.1332 M.
Read more: Uses of acetic acid
Advantages of Molarity
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The advantages of molarity are as follows:
- The major advantage of molarity is that it is easy and convenient to use because the solute may be measured in grams, converted into units of moles and then mixed with a volume.
- Stoichiometry calculations are simplified as moles are used rather than mass.
- Amount of solutions is measured by volume which makes titration easy.
- Permits calculation of density and ionic strength.
Disadvantages of Molarity
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The disadvantages of molarity are as follows:
- Molarity is dependent on temperature. It changes if the solvent is too hot or cold.
- The exact amount of solvent in a given volume cannot be determined if density is not known.
Molarity vs Molality
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People often confuse Molarity with Molality whereas there is a significant difference among them. The difference between Molarity and Molality are as follows:
| Basis of Distinction | MOLARITY | MOLALITY |
|---|---|---|
| Meaning | The ratio of moles of solute to the volume of solution in liters. | The ratio of solute to the volume of solvent in kilograms |
| Units of Measurement | Mol/L | Mol/kg |
| Dependence |
| Volume of Solvent. Does not depend on pressure or temperature. |
| Symbols | Denoted by “M”. | Denoted by “m”. |
| Formula | M=moles of soluteVolume of Solution in L | m=moles of soluteVolume of solvent in kg |
Things to Remember
Before solving sums on Molarity using the Molar Concentration Formula, we must remember the following things:
- The molarity formula is: M =nV where M is Molarity, n is moles of a solute and V is the Liters of solution.
- SI unit of Molarity is Mol/m3 .
- When the unit of solution is given in units other than Liters, it must be converted to Liters.
- Mixtures with the same uniform composition are known as Homogenous Mixtures.
- Mixtures with non-uniform composition are known as Heterogeneous Mixtures.
- A solution in which the solvent is water is known as Aqueous Solution.
Sample Questions
Ques 1. Calculate the molarity of a solution containing 5 g of NaOH in 450 mL solution. (4 marks)
Ans. Moles of NaOH = -1.5 g40 g mol-1 = 0.125 mol
Volume of the solution in liters = 450 mL / 1000 mL L-1
Using equation M=nV we get,
Molarity = –1 0.125 mol × 1000 mL450 mL L-1
= 0.278 M
= 0.278 mol L-1
= 0.278 mol dm–3.
Ques 2. A solution is prepared by bubbling 1.56 grams of hydrochloric acid in water. Here, the volume of the solution is 26.8 mL. Calculate the molarity of the solution. (10 marks)
Ans. The chemical formula of hydrochloric acid = HCl
The chemical formula for Water = H2O
The molecular weight of HCl = 35.5 ×1 + 1×1 = 36.5 moles/gram
The molecular weight of H2O = 1 ×2 + 16 ×1 = 18 moles/gram
Given, mass of hydrochloric acid in the solution = 1.56 g
The number of moles of hydrochloric acid = mass in grams molecular weight
= 1.5636.5 = 4.27 × 10-2 moles
Now, given volume of the solution = 26.8 mL
Expressing the volume in terms of liters, we get,
Volume = 2.681000 = 2.68 × 10-2 L.
Now we know that,
Molarity=number of moles volume of solution in liters
=4.27×10-22.68×10-2
=1.59 M
Therefore, the molarity of the solution is 1.59 M.
Ques 3. If 16.35 mL of an AgNO3 solution reacts with 0.0003017 kg of KIO3, find its concentration. (5 marks)
Ans. AgNO3 + KIO3 = AgIO3 + KNO3
Number of moles in KIO3 = Mass/Molar mass
n=0.3017g214.0g mol-1
=0.00141mol
Hence,
Number of moles in AgNO3 = 0.00141 mol
Now,
Concentration of AgNO3 (M) = Moles/Volume
M =0.001410.01635
= 0.08624 M
Ques 4. Find the molarity of 4 g of caustic soda, if it was in 200 ml of solution. (4 marks)
Ans. Mass = 4 g
Volume = 200 ml
Molar mass of caustic soda = 23 + 16 + 1 g
= 40 g
Molarity = (Mass/Molar Mass)/Volume
= (440)/(1000200)mol L-1
= 0.5 M
Ques 5. 15 g sodium sulphate was used to make a solution. The solution has a volume of 125 ml. Find the molarity of the sodium sulphate solution supplied. (4 marks)
Ans. Molar mass of sodium sulphate = (23 × 2) + 32 + (16 × 4) = 142 g
Number of moles in sodium sulphate = 15/142 = 0.106
Volume of the solution (V) = 0.125 L
Thus,
Molarity (M) = nV
= 0.1060.125
= 0.85 M
Ques 6. Determine the molar concentration of sodium hydroxide for its reaction with hydrochloric acid. (4 marks)
Ans. HCl + NaOH → NaCl + H2O
Number of moles in HCl = Number of moles in NaOH = 8.75 × 10-3 mol.
Volume of solution = 25 × 10-3L
Therefore,
The Molar Concentration of NaOH (M)
M = 8.75 × 10−3/25.0 × 10−3
= 0.350 mol L-1
Ques 7. How many grams of NaCl are contained in 0.250 L of a 5.30-M solution? (5 marks)
Ans. We know that,
M = mol of soluteL of solution
Or, mol of solute=M ×L of solution
=5.30 ×0.250mol
=1.325 mol of NaCl
Therefore,
Mass of NaCl=1.325 ×58.4425g
=77.4g of Nacl
Ques 8. A 355-mL soft drink sample contains 0.133 mol of sucrose (table sugar). What is the molar concentration of sucrose in the beverage? (2 marks)
Ans. Since the molar amount of solute and the volume of solution are both given, the molarity can be calculated using the definition of molarity. Per this definition, the solution volume must be converted from mL to L:
M = mol of soluteL of solution
={0.133mol}/355mL×1L1000mL
=0.375M
Ques 9. What is the molarity of pure water assuming its density 1 gm/cc ? (5 marks)
Ans. Pure water is one component system.
According to definition,
Molarity = M =nV
Let, V = 1 liter.
The mass of 1 liter of water is 1000 gm and density is 1 gm/cc.
The number of moles in 1000 gm is :
n = 1000/18
= 55.56
Putting this value in the formula of Molarity, we get,
M =nV
=55.561
= 55.56
Ques 10. Determine the molarity of 750cc NaOH solution containing 40 gm NaOH. (5 marks)
Ans. Molecular weight of NaOH = 23+16+1 = 40.
Hence,
n = 40/40 = 1
V=750cc
= 750/1000 L
Now,
Putting values of n and V in the molar concentration formula, we get,
M =1750X1000
= 1.33M
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