How do you work out moles?I need an explanation. My teacher isn't helping me on it...

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amy-lepore eNotes educator| Certified Educator

How many moles of silicon are in 4.60 g of Si? This is the first venture into one of the most common chemical calculations, finding the number of moles present in a given mass of substance. Restate it: 4.60 g Si <=> ? mol Si                         The atomic mass (you need this for each element in your problems, too) of silicon, 28.09 (rounded from 28.0855), gives us the needed tool, because we know from the definition of the mole that the following is true about silicon.            28.09 g Si <=> 1 mol Si     Like any equivalency, this one makes available two conversion factors.  One will be the tool we need for our grams-to-mole conversion.  28.09 g Si/1 mol of Si and 1 mol Si/28.09 g Si or:

28.09 g Si  and   1 mol Si

1 mol Si           28.09 g Si

Thus if we multiply the given, 4.60 g Si, by the second conversion factor, we will obtain the answer. Solution:  We carry out the following calculation.  Notice how the units, g Si, cancel to leave the correct unit, mol Si (Draw in the cancel lines yourself).

4.60 g Si x  1 mol Si/28.09 g Si  = 0.164 mol Si

In other words, 4.60 g Si <=> 0.164 mol Si.  Now, check to see if the answer makes sense.  Is its numerical size in the right range?  Yes, because we began with 4.60 g Si, clearly less than a whole mole of silicon, so the answer has to reflect this obvious fact. The atomic mass=1 mole.

enotechris eNotes educator| Certified Educator

A mole (otherwise known as "Avogadro's Number," first determined by Italian scientist Amadeo Avogadro (1776-1856) ) is a specific, extremely large number, like a million (1 * 10^6) or a billion (1* 10^9).  If you count atoms, molecules, marbles, or cars, and have 6.02 * 10^23 of them, you've got a mole. Consider it just like any ordinary number with which you can add, subtract, multiply, or divide. If you take the atomic mass of an element, which is unitless, and specify that atomic mass as a number of grams, by definition, you've got 1 mole of that element. If you've got a solid of so many grams, you can calculate the number of moles by remembering that grams / grams per mole = # of moles. If you've got a gas, the constant to remember is 22.4 liters / mole.

chezmamann | Student

How you work out moles depends on the question you are asked. The mole of any element is simply its atomic weight. Thus, one mole of carbon is 12.01 grams because that is the atomic weight of carbon. In one mole of oxygen there is 16.00 grams because that is the atomic weight of oxygen. If you are asked to find out how many moles there are in a given mass you use the equation: number of moles = mass (given)/atomic weight.

Eg: how many moles of carbon is 28 g of carbon ?

To work this out you use the above formula. Mass= 28. Atomic weight = 12.01

Answer= 28/12

= 2.33 moles of carbon.


One mole of any element has 6.022 x 10 to the power of 23 atoms ( Avogadro's number). Thus in one mole fluorine there are 6.022 x 10 to the power of 23 atoms. In one mole of Gold there are 6.022 x 10 to the power of 23 atoms.To find out the number of atoms you use the formula: number of atoms = number of moles x Avogadro's number.

Eg: How many atoms of silver in 2 moles of silver.

Answer: 2 (number of moles) x 6.022 x 10 to the power of 23

Sometimes when you ask what time it is, some people feel the need to yell you how to build a watch.

There is a very Simple way to understand calculating Mols, without all the extraneous talk about "conversion factors" (sometimes explanations by educators are not meant to "explain", but to prove how smart they sound with " substituting, factoring and formulas).

Try this:

The # of Mols is...

If the substance's mass is Greater than molecular weight, divide  mass by molecular weight. (Gives you Mols)

If the substance's mass is Less than the molecular weight, divide molecular weight by mass. (Gives you Mols).

Now how hard was that?

Simple... Done!

(Quick mention: with solutions, mixtures and alloys etc.. You must take into account the percentages of each element.)

Wiggin42 | Student

A mole of anything means to have 6.02 x 10^23 of that item. Think of it like a dozen. A dozen eggs means to have 12 eggs. If a question asks you how many eggs are in two dozen, you'd say 24. A mole works the same way but on a larger scale. 

Jyotsana | Student

The formula for Moles are:

Moles=Mass/Molar Mass

Mass=Moles*Molar Mass

Moles=number of atoms/molecules / Avogadro # 6.023*10^23

#atoms or molecules= moles*Avogadro# 6.023*10^23

You just have to plug the number that's given and solve it.

parama9000 | Student

Sorry it shld be Mols! My bad.

parama9000 | Student

You really just need these 4 formulas:

Molecules= Mass/ Molar mass

Mass= Moles x Molar mass

Moles= number of atomsx6.23x10^23

Number of atoms= Moleculesx6.23x10^23

parama9000 | Student

Are moles molecules?

Yojana_Thapa | Student

To work out moles, all you need is the formula.

The formulas that has to do with moles are:

  • Moles = Mass/ Molar Mass
  • Mass = Moles x Molar Mass
  • Moles= # of atoms/Molecules / Avogadro # (6.023 x 10 ^23 )
  • #atoms or molecules = Moles x Avogadro # (6.023 x 10 ^23 )

To find moles, the question would be like this

  1. Calculate # of moles in 20 g CH4

- Mass= 20g

- Molar mass = (1 x 12.01) + (4 x 1)= 16.01

- 20 g / 16.01g/mol

- Moles = 1.25