Quantitative chemistry revision

Relative Formula Mass

A relative formula mass is just the relative atomic masses of all of the atoms in the molecular formula ADDED TOGETHER.

For example, find the relative formula mass of MgCl2

The atomic mass of Mg = 24 and the atomic mass of Cl = 35.5 (use the periodic table)

Then you just add together the masses - Mg + (2 x Cl) = 24 + (2 x 35.5) = 95

Therefore, the relative formula mass of MgCl2 = 95

You can also calculate the percentage mass of an element in a compound which can be calculated by

Atomic mass x number of atoms of that element. x 100
Relative formula mass of the compound

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The mole is simply the name given to an amount of a substance. One mole of any substance is just an amount of that substance that contains an Avogadro number of particles

Avogrado constant = 6.02 x10 (power of 23)

Here is a formula to find the number of moles in a given mass:

Number of moles = mass in g (of an element in a compound)
Mr (of the element or compound)

EG: How many moles are there in 66g of carbon dioxide??

Mr of CO2 = 12 + (16 x 2) = 44

No. of moles = mass (g) / Mr = 66 / 44 = 1.5 mol

No of moles x Mr

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Conservation of Mass

In a chemical reaction, mass is always conserved.

During a chemical reaction NO ATOMS ARE DESTROYED, and NO ATOMS ARE CREATED. This means there are the same number and types of atoms on each side of a reaction equation.

For example, 2Li + F2 - 2LiF

In this reaction, there are 2 lithium atoms and 2 fluorine atoms on each side of the equation. By ADDING up the relative formula masses of the substances on each side of a balanced symbol equation, you can see that mass is conserved. The total Mr of all the reactants EQUALS the total Mr of the products.

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The Mole and Equations

You can balance equations using reacting masses.

If you know the masses of the reactants and products that took part in a reaction, you can work out the BALANCED SYMBOL EQUATION for the reaction. Here are the steps you should take:

Step 1: divide the atomic mass of each substance by it’s relative formula mass to find the number of moles.

Step 2: divide the number of moles of each substance by the SMALLEST number of moles in the reaction.

Step 3: if any of the numbers are not whole numbers, multiply ALL the numbers by the same amount so that they ALL become WHOLE numbers.

Step 4: write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulaes.

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Limiting Reactants

Reactions STOP when ONE reactant is USED UP.

When a solution is reaction in acid it produces bubbles and gas which signals that it is reacting. After a while, the amount of fizzing slows down and the reaction eventually stops.

The reactions stops when all of one of the reactants is used up. Any other reactants are in excess. They’re usually added in excess to make sure that the other reactant is used up.

The reactant used up is called the LIMITING REACTANT.

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Limiting Reactants 2

You can calculate the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation using these steps,

1) Write out the BALANCED EQUATION.

2) Work out the relative formula masses of the reactant and product you want.

3) Find out how many MOLES there are if the substances you know the mass of.

4) Use the balanced equation to work out how many moles there’ll be of the other substance. In this case, that’s how many moles of product will be made of this many moles of reactant.

5) use the number of moles to calculate the mass.

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Concentration Calculations

You can calculate concentration of a substance in two ways, dependent on which units you are using.

If you are using grams,

concentration = Mass of solute
Volume of solvent

If you are using moles,

Concentration = number of moles of solute
Volume of solvent

The more solute (the substance that’s dissolved) there is in a given volume, the more concentrated the solution. You can also use the concentration triangle,

No. of moles
Concentration x volume

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Atom Economy & Percentage Yield Calculations

ATOM ECONOMY - a lot of reactions make more than one product. Some of them will be useful but others will be waste. The atom economy of a reaction tells you how much of the mass of the reactant is wasted when manufacturing a chemical and how much ends up as useful products. This is the equation:

Atom economy = Relative formula masses of desired products x100

Relative formula mass of all reactants.

100% atom economy means that all the atoms in the reactants have been turned into useful (desired) products. The higher the atom economy the greener the process.

PERCENTAGE YIELD - the amount of you get is known as the yield. The more reactants you start with, the higher the actual yield will be. Percentage yield is given by the formula:

Percentage yield = mass of product actually made. x100
Maximum theoretical mass of product

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