What is the mole ratio?
A mole ratio is the ratio between the amounts in moles of any two compounds involved in a balanced chemical reaction. The balance chemical equation provides a comparison of the ratios of the molecules necessary to complete the reaction. We cannot calculate mole ratio for an unbalanced equation.Mole ratios are used as conversion factors between products and reactants in many chemistry problems.
Table of contents
- What is a mole?
- Determination of mole ratio in stoichiometric reaction
- Unit of mole ratio
- Problem practice on mole ratio
- FAQs
What is a mole?
In chemistry the mole is a fundamental (SI) unit used to measure the amount of a substance. This quantity is sometimes referred to as the chemical amount.
A mole is defined as the amount of a substance that contains exactly 6.022 x 1023 ‘elementary entities’ of the given substance.
A mole contains 6.023 x 1023 units. These units can be atoms, molecules, ions, electrons or anything else.
Examples:
- 1 mole of hydrogen atom = 6.023 x 1023 hydrogen atoms
- 1 mole of hydrogen molecule = 6.023 x 1023 hydrogen molecules.
- 1 mole of electrons means = 6.023 x 1023 electrons.
- 1 mole of sodium ions = 6.023 x 1023 sodium ions.
How much does one mole weigh?
The mass of one mole atom of any element is exactly equal to the atomic mass in grams of that element.
Similarly, the mass of 1 mole molecule of any substance is equal to its molecular mass in grams or gram molecular mass.
Examples:
- Mass of 1 mole hydrogen atom = 1 g
- Mass of 1 mole of hydrogen molecule = 2 g
- Mass of 1 mole of oxygen atom = 16 g
- Mass of 1 mole of water (H2O) molecule = 18 g
∴ Mole of the substance = (Mass of the substance in gm / Gram molecular mass of that substance)
Also read ⇒ Mole concept
Determination of mole ratio in stoichiometric reaction
Stoichiometry is an important concept in chemistry that helps us use balanced chemical equations to calculate amounts of reactants and products. Here, we make use of ratios from the balanced equation. In general, all the reactions that take place are dependent on one main factor, how much substance is present. We cannot calculate the mole ratio for an unbalanced equation.
1. Let us consider a balanced chemical equation
N2 + 3H2 → 2NH3
- Here the coefficients of N2 =1
- The coefficients of H2 = 3
- The coefficients of NH3 = 2
This coefficient provided the molecular information for the chemical equation.
In this chemical equation 1mole of nitrogen (N2) reacts with 3 moles of hydrogen (H2) to produce 2moles of ammonia (NH3).
To determine the mole ratio of any two components of the equation, compare the coefficients of each component in the equation.
- Mole ratio between N2 and H2 = (1 mole of N2 / 3 mole of H2) = 1:3
- Mole ratio between N2 and NH3 = (1 mole of N2 / 2 mole of NH3) = 1:2
- Mole ratio between H2 and NH3 = (3 mole of H2 / 2 mole of NH3) = 3:2
- Mole ratio between H2 and N2 = (3 mole of H2 / 1 mole of N2) = 3:1
- Mole ratio between NH3 and N2 = (2 mole of NH3 / 1 mole of N2) = 2:1
- Mole ratio between NH3 and H2 = (2 mole of NH3 / 3 mole of H2) = 2:3
2. Consider the balanced reaction
2H2 + O2 → 2H2O
- Here the coefficients of N2 =2
- The coefficients of O2 = 1
- The coefficients of H2O = 2
In this reaction 2 mole of H2 react with 1 mole of O2 producing 2 moles of H2O.
- Mole ratio between H2 and O2 = (2 mole of H2 / 1 mole of O2) = 2:1
- Mole ratio between H2 and H2O = (2 mole of H2 / 2 mole of H2O) = 2:2
- Mole ratio between O2 and H2O = (1 mole of O2 / 2 mole of H2O) = 1:2
Unit of mole ratio
Mole ratio is a dimensionless unit. Because it produces mole:mole, so units of moles are cancelled out.
Problem practice on mole ratio
1. How many moles of Fe molecules will react with 0.27 moles of O2 molecules in the given equation?
4Fe + 3O2 → 2Fe2O3
Solution: In the given chemical equation 4Fe + 3O2 → 2Fe2O3
4 moles of Fe react with 3 mole of O2 produces 2 mole of Fe2O3
Mole ratio of Fe and O2 Fe : O2 = 4:3
Let the 0.27 moles of O2 react ‘x’ mole of Fe molecule, now Fe : O2 = x : 0.27
By comparing both we can find out x value
4:3 = x : 0.27
⇒ x = {4 * (0.27)} / 3 = 0.36 mole
∴ 0.36 mole of Fe molecules will react with 0.27 mole of O2.
2. How many moles of ammonia are produced if 4.20 moles of hydrogen are reacted with an excess of nitrogen?
Solution: The balanced equation of formation of ammonia is
N2 + 3H2 → 2NH3
Here 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia.
For 4.20 mole of hydrogen produces the number of moles ammonia is
From the equation the mole ratio of hydrogen and ammonia is H2 : NH3 = 3:2
It means 3 moles of H2 produces 2 moles of NH3
Let 4.20 moles of H2 will produce ‘x’ moles of NH3 . Now H2 : NH3 = 4.20 : x
By comparing both we can find out x value
3 : 2 = 4.20 : x
⇒ x = 4.20 (⅔) = 2.8 moles
∴ 4.20 moles of H2 will produce 2.8 moles of NH3
Frequently Asked Questions on Mole ratio
How do you find the mole ratio?
A mole ratio is the ratio between the amounts in moles of any two compounds involved in a balanced chemical reaction
What is the mole ratio between N2 and H2 in NH3?
In the balanced chemical equation of ammonia is N2 + 3H2 → 2NH3, the mole ratio between N2 and H2 is 1:3
What is the mole ratio of zinc to sulfuric acid?
In the balanced chemical equation of zinc to sulfuric acid is Zn + H2SO4 → ZnSO4 + H2, the mole ratio between Zn and H2SO4 is = 1:1
Why are mole ratios important?
Mole ratios allow comparison of the amounts of any two materials in a balanced equation. Calculations can be made to predict how much product can be obtained from a given number of moles of reactant.
What is stoichiometry and why is it important?
Stoichiometry is an important concept in chemistry that helps us use balanced chemical equations to calculate amounts of reactants and products. Here, we can use mole ratios from the balanced equation.
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