# Chemistry DF

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• Created by: shiji
• Created on: 15-10-12 19:27

Calculations:

How do you work out 1 mole of a compound?

You add up all the Ar values to give you the Mr J E.g H2O = (1x2) +16 = 18

Reacting Masses:

This is when you are asked to figure out the mass of a product formed or reactant used up.

Example:

What mass of magnesium oxide is produced when 1.2g of magnesium reacts with carbon dioxide?

Mg = 24.3 and O = 6.0

Step 1: Write out a balanced equation and underline the substances involved.

Step 2: under each substance write the amount of moles that is given.

Step 3: Calculate the masses of each substance

Step 4: You divide the substance whose mass you are given in the equation by itself so it equals 1g in this situation it’s the reactant. You then divide the other substance by the other mass and multiply by the mass given in the equation.

Volumes of gases:

Important to remember that 1 mole of any gas at room temperature is always 24dm3

Really similar to reacting masses except the will ask for the volume of a gas instead

Example:

What volume of Carbon Dioxide is produced when 2.0g of Calcium Carbonate is heated?

Step 1: Write out a balanced equation:

Step 2: Write out the moles involved in the reaction

Step 3: Write out the masses and volumes involved (Mr of product and 24dm3 for gas)

Step 4: Divide the substance that is not a gas by itself to give 1 and then divide the gas by that substance mass and multiply by the mass given in the equation J

Sometimes you will get a different question involving gases where you are asked to calculate how much volume is occupied by a certain mass of a gas.

Example:

Calculate the volume occupied by 4.4g of carbon dioxide at room temperature and pressure

Step 1:  Calculate the amount of moles using the equation mass(from equation)/ Mr

Step 2: If 1 mole = 24 dm3 then you are able to scale your answer accordingly.

You may also be asked to calculate the mass of a volume of gas

Example:

Calculate the mass of 1.2dm3 of methane gas at room temp and press

Step 1: Calculate the amount of moles using the equation of: volume of moles/molar volume

Step 2: You multiply your answer from above by the molar mass of the gas and voila J

Enthalpy Changes:

These are pretty simple as you just treat it like a reacting mass equation!

Example:

The enthalpy of combustion for methane is -890kJmol-1 Calculate the energy released when 3.2g of methane is burnt completely.

Step 1: Write a balanced equation and underline. Remember you need the enthalpy change at the side!

Step 2: Indicate moles involved. (only needed for the substance)

Step 3: Calculate the mass of moles

Step 4: Divide this