How To Perform Simple Calculations on Mole

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A mole is the amount of a substance which contains as many elementary entities such as atom, molecules and ions as contain in 12g of carbon-12.


The mole concept is usually illustrated by series of calculation based problems. Any substance at standard temperature and pressure (stp) usually is a mole representation in an uncombined state. 1mole of a substance at STP contains the Avogadro’s constant ( 6.02×1023 Units).


Mathematically,  mole is  expressed as the ratio of mass concentration in grams  over the molar mass of a substance in grams per mole.Students also check out How to carry out titration experiment


Mole       =    Mass concentration in grams / Molar mass in
g/mol Therefore the unit of Mole is mol or m
Example 1
What is the no of ions present in 12g of sodium chloride solution?


Solution
Mole = mass/molar mass
Molar mass of NaCl  = 23 + 35.5 = 58.5g/mol
Mass = 12g
Therefore, mole = 12g/58.5g/mole = 0.205mol
The no of ions = mole  x Avogadros constant 0.205mol x 6.02×1023 Units = 1.2717×1023 ions



Example 2
How many moles are there in 0.326 g of barium hydroxide?
Ba(OH)2 = 171. That is, one mole of barium hydroxide has a mass of 171 g. As we have
less than 1 gram, we clearly have less than one mole – we’re looking for a
small answer!

Number of moles = 0.326/171 = 0.00191 mol


Working with reacting masses – using equations


We usually work out the number of moles of a substance so that we can then work
out the number of moles of another substance which is reacting with the first.
In order to do this we need to look at the equation for the reaction. Again, don’t write down too many significant figures, but do use them all in your calculations. This type of calculation is important if we are planning the quantities of reactants to use in an experiment.



Example 1
What mass of iron reacts with 10.0 g of sulphur? [Fe = 56, S = 32]
First we need the correctly balanced equation:
Fe(s) + S(s) ® FeS(s)
The equation shows that one mole of sulphur reacts with one mole of iron. We always
need the same number of moles of iron and sulphur. We can work out the number
of moles of sulphur from the mass:


Number of moles of sulphur = 10.0/32 = 0.313 mol

Number of moles of iron = 0.313 mol (the same as the sulphur)

Mass of iron = 0.313 × 56 = 17.5 g


Example 2
What mass of magnesium oxide is made when 250 g of oxygen reacts with excess
magnesium? It is important to have an excess of magnesium so that all of the
oxygen reacts. [Mg =24, O =16].
2Mg(s) + O2(g) ® 2MgO(s)
The equation shows that 1 mole of oxygen produces two moles of magnesium oxide.
There will always be twice as many moles of magnesium oxide as oxygen (or half
as many moles of oxygen compared to magnesium oxide).
Number of moles of oxygen = 250/32 = 7.81 mol

Number of moles of magnesium oxide = 2 × 7.81 = 15.6 mol (twice as many)

MgO= 40

Mass of magnesium oxide = 15.6 × 40 = 625 g

Study the following:


What is a Standard Solution

How to prepare a Standard Solution of 0.1M Concentration of Sodium hydroxide

What are homologous series

What are Conjugate Base

What Are Organic Compounds ?

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