Counting Chemical Substances In Bulk :) 

You should know by now how to calculate the molecular mass of a molecule and a relative atomic mass. 

When we take the relative molecular mass or relative atomic mass of a substance in grams, we say we have one mole of the substance. The mole is the chemist's unit of amount.

The definition of a mole is: a mole of a substance is the mass of the substance that has the same number of particles as there are atoms is exactly 12g of carbon-12. The particles may be atoms, molecules, ions or even elements.

The number of atoms or molecules in one mole is a constant known as Avogardo's Constant. Avogardo's constant, NA, is 6.02x1023 mol-1.

To find the amount of substance present in a given mass, we must divide that mass by the molar mass, M, of the substance. For example, for NaCl, M=23.0+35.5=58.5gmol-1; so in 585g of sodium chloride (NaCl) there are 585/58.5 mol of NaCl, i.e 10 mol NaCl.

To find the mass of a given amount of substance, we multiply the number of moles of the substance by the molar mass. For this you can use an equation mass=number of moles x molecular mass. This equation can be easily rearranged to work out the thing you want.

Worked examples:

Calculate the mass of the following:

a) 0.1 mol of carbon dioxide

b) 10 mol of calcium carbonate, CaCO3.

To work out the mass of 0.1 mol of carbon dioxide, first you have to find out the molecular mass of carbon dioxide. Carbon dioxide has a formula of CO2. Now that you have the formula, you can look at the periodic table and find C and O. Carbon can be found in the 4th group and it has an atomic mass of 12. Oxygen can be found in the 6th group and has an atomic mass of 16. You need one carbon and two oxygens so the equation will look like this:    12+(2 x 16) = 12 + 32 = 44. So now you know that the molecular mass of carbon dioxide is 44. Now to work out the mass of carbon dioxide you will need to use the equation: mass = number of moles x molecular mass. So the mass of carbon dioxide is: 0.1 x 44 = 4.4g.

 

To work out the mass of 10 mol of calcium carbonate you will have to do the same thing as in the example above. First calculate the molerular mass:    Ca + C + O x 3 = Mr. Put the number into the equation: 40 + 12 + (16 x 3) = 40 + 12 + 48 = 100. Now you need to timse the moles and molecular mass together: mass of CaCO3 = 10 x 100 = 1000g.

 

This free website was made using Yola.

No HTML skills required. Build your website in minutes.

Go to www.yola.com and sign up today!

Make a free website with Yola