• # Category:Standard Grade Chemistry Revision Notes

## Calculations (Credit).

There are a couple of calculations that you need to be able to do in your Standard Grade Chemistry exam. They are simple, but easily forgotten.

--LuhLah 17:58, 2 August 2009 (BST)

Formula Mass

Before attempting these calculations, you must: a. Know how to write chemical formula b. Know what chemical formula means.

For example, the chemical formula for carbon dioxide is CO2. This means there is one carbon atom and two oxygen atoms in every chemical formula of CO2.

c. Know where to find the relative atomic masses (RAM) of the elements.

These are found on Page 4 of the data booklet.

[u]Worked Example[/u] Calculate the formula mass of carbon dioxide.

Answer: Write down it's chemical formula CO2 Turn to Page 4 of the data booklet Write down the RAM for carbon and oxygen, and then multiply by the number of atoms there are in the formula.

[itex] C = 12 X 1 = 12 O = 16 X 2 = 32[/itex] Add the numbers together, and this will give you the formula mass of CO2 [itex]12 + 32 = 44 [/itex]

BUT, sometimes chemical formulae have brackets...

[u]Worked Example[/u] Calculate the formula mass of sodium carbonate.

Answer: Write down it's chemical formula Mg(NO3)2 Turn to page 4 of the data book. Write down the RAM for magnesium, nitrogen & oxygen and multiply by the number of atoms there are in the chemical formula. NB: the number outside the bracket multiplies each element inside the bracket as shown bellow: [itex] Mg = 24.5 x 1 = 24.5 N = 14 x 1 x 2 = 28 O = 16 x 3 x 2 = 96 [/itex] Add the numbers to get the f.m [itex]24.5 + 28 + 96 = 148.5 [/itex]

FM & The Mole

Concentration & The Mole

Calculations with both triangles

Titration Calculations

Percentage composition by mass

Calculations on balanced equations

## Diatomic Elements

As you should know, "di" means two and "atomic" means atom. So, n diatomic element is an element which exists as a molecule of two atoms. Below are the diatomic elements you need to know for Standard Grade:

Hydrogen, Iodine, Nitrogen, Chlorine, Bromine, Oxygen and Fluorine.

A simple way to remember these is why writing HINClBrOF. These are the initials of the diatomic elements, and you need to know this mean formula writing, and sometimes doing calculations.

## Valency

There are a few ways of figuring out the valency of said substances.

Elements

Simply look at the group number that the element is in: this is the valency. But, don't fall into the trap where say, Nitrogen is in group 5, therefore you think Nitrogen has a valency of 5. It works as below:

Group 1: Valency 1 Group 2: Valency 2 Group 3: Valency 3 Group 4: Valency 4 Group 5: Valency 3 Group 6: Valency 2 Group 7: Valency 1 Group 8/0: No valency (Nobel gases are monatomic)

Ions

The valency of an ion is the charge number. For example, from the SG/Int 2 Databook, the table of selected ions tells us that sulphate has a charge of "2-", this means that it's valency is 2.

This can be really helpful when working out the formula of acids. For this, you will need to know that hydrogen is always in the acid formula and has a valency of 1, and you figure out the valency of the ion. (eg sulphuric acid, we know it contains "sulphate", so we get the formula/valency from the table). For example, the formula for nitric acid is HNO3. To get this, we know that Hydrogen is in the formula and has a valency of 1, and we can see that nitrate has the formula NO3-, so it's valency is also 1, giving HNO3.

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