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Mole calculator

28 Sep 2015Homework Help Online

Mole calculator is a great help in chemistry. The online mole calculator is used for computing the average molecular weight of molecules by entering the chemical formulas. First of all, the mole is a unit which measures the amount of substance. Mole calculations are very common in chemistry.  The calculator like below can handle at most two different bracket levels.



How to calculate the mass from the number of moles?

This can be explained with an example. Taking CaCO3, if this is added, the molar mass is 100 g mol-1, which means one mole has a mass of 100 g. If we calculate 2 mole, clearly two moles weigh 200 g (2x100g).

Mass of substance (in g) = number of moles (in mol) x mass of one mole (in g mol-1)

How to calculate the number of moles from the mass?

Again, the above equation can be rearranged.

Number of moles (in mol) = Mass of substance (in g)/ mass of one mole (in g mol-1)

If we take the above example, then using CaCO3 which had a molar mass of 100 g mol-1 and if we have 200 g of CaCO3, then the answer is 2 moles (200/100 = 2).

Or in other words, we can say that dividing the mass by the mass of one mole gives the number of moles.


How many moles are there in 0.326 g of barium hydroxide?

Ba(OH)2 = 171.

This shows that 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.

Using the equation:

Number of moles (in mol) = Mass of substance (in g)/ mass of one mole (in g mol-1)

Number of moles = 0.326/171

Answer: 0.00191 mol

The quantities of the chemicals can also change and here we work with reacting masses. First we calculate the number of moles of one substance and then of the other substance which is reacting with the first.

For example:

What mass of iron reacts with 10.0 g of sulphur? [Fe = 56, S = 32]

First, balance the equation:

Fe (s) +S (s) = FeS (S)

The equation shows that one mole of sulphur reacts with one mole of iron. We need a balanced equation, so the moles of sulphur and iron must be same. We can calculate 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

Answer: 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

Answer: 625 g

Some useful links:

  1. HalesOwen site is very useful in understanding the depth of mole calculator as it has detailed examples to and the exercises to practice.
  2. Here you will find some useful definitions to understand the concepts of chemistry can be studied here.
  3. Another chemical mole gram calculator is given on this site. You can calculate and can practice on the mole   calculator.


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