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Number of Moles Formulas - Avogadro’s Constant
The mole is a measure or the base unit for the amount of substance present in the given sample. 1 mole is a number that is equal to 6.022 x 1023 particles, also known as the Avogadro’s constant. These particles can be any type of species, e.g atoms, molecules, electrons, protons, neutrons, etc. In fact, if we have a which contains approximately 6.022 x 1023 almonds, it can be said that we have 1 mole of almonds in that particular box.
This number, 6.022 x 1023 (Avogadro’s constant) is actually defined based on the number of atoms present in 12g of 12C isotope of Carbon. Where 12gm is the molar mass of 12C or in other words the mass of 1 mole or 6.022 x 1023atoms of 12C isotope of Carbon.
What is the Mole Concept: Understanding the Number of Moles
The mole concept is a simpler method for expressing the amount of a substance in chemical substances. For this, we have two steps, the first is the numerical magnitude and the other is the units in which the magnitude is expressed.
Now, let us say that the mass of a bucket is 5 kilograms, here 5” is the magnitude and the unit is ‘kilogram’. However, while dealing with the particles at an atomic or molecular level, we do not find this traditional method suitable. Like even one gram of a pure element comprises a huge number of atoms, which is the reason we are using the mole concept.
Here, the mole concept mainly focuses on the unit ‘mole’. This concept pertains to the count of a very large number of particles. Here, one mole of any substance equals the Avogadro number. The Avogadro number has a value of 6.023×1023. This value is very useful for measuring the products in any chemical reaction. Thus 6.023×1023 of atoms, molecules or particles are 1 mol of atoms, molecules or particles.
Formula for the Number of Moles Calculation
Now, in order to calculate the number of moles of any substance present in the sample, we simply divide the given weight of the substance by its molar mass.
The number of moles formula is:
\[ n= \frac{\text{mass of substance}}{\text{mass of one mole}} \]
Mathematically,
\[ n = \frac{m}{M}\]
Here,
n = The number of moles,
m = The given mass, and
M = The molar mass
Solved Example on the Number of Moles of a Substance
Example 1: How many moles are in 25.0 grams of water?
Solution:
In order to find the number of moles, we will have to use the following formula:
n = m/M
Molar Mass, M, of H2O = 18.0 g/mol
Given mass, m, of H2O = 25.0 gm
So, n = 25.0 (gm) / 18.0 (gm/mol)
Number of moles, n = 1.39 mol
Example 2: How many moles are in 3.4 x 1023 molecules of H2SO4?
Options:
(a) 0.82 moles
(b) 0.01 moles
(c) 5.56 moles
(d) 0.56 moles
Answer: (d)
Solution:
1 mole of anything = 6.022x 1023 of anything
In order to find the number of moles here, we will have to divide the given number of particles by Avogadro’s constant.
So, n = 3.4 x 1023 / 6.022 x 1023
Then number of moles, n = 0.56 moles
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FAQs on Number of Moles Formula
1. How do you find the number of moles?
We can determine the number of moles of a compound with a known mass.
So, to determine the number of moles in a sample, we simply weigh it and divide the measured weight by the molecular weight. The quotient we obtain here is equal to the number of moles.
2. How do you find moles from molarity?
In order to calculate the number of moles in a solution if we are given the molarity, we simply multiply the molarity by the total volume of the solution in litres to get the number of moles in a solution.