# Chemistry C3 Calculations

Calculations needed for AQA C3 Chemistry

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• Created by: Charlotte
• Created on: 15-05-10 14:47

## Titration Calculations - Finding a concentration i

EXAMPLE

25cm^3 of sodium hydroxide of concentration 0.1 moles / dm^3. It takes 30cm^3 of sulfuric acid to neutralise. Find the concentration in moles/dm^3.

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## Continued

1: Find moles of 'known' substance. moles = concentration x volume.

0.1 x (25/1000) = 0.0025 moles of NaOH

2: Write balanced equation to find how many moles of 'unknown' substance you must have had.

2NaOH + H2SO4 --> Na2SO4 + 2H2O

For every 2 moles of NaOH there was one mole of H2SO4. Therefore 0.0025 moles of sodium hydroxide is...

0.0025/2 = 0.00125 moles of H2SO4

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## Continued

3: Find concentration of 'unknown' substance. Concentration = number of moles/volume

= 0.00125/(30/1000)

= 0.0417 mol/dm^3

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## Titration Calculations - Finding a concentration i

EXAMPLE (carried on from previous calculation)

1: find RFM (Relative Formula Mass) of acid

H2SO4 = (1 x 2) + 32 + (16 x 4) = 98

2: convert from moles into grams. mass in grams = moles x RFM

= 0.0416666... x 98 = 4.08333... g

= 4.08g/dm^3

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## Calorimetry Calculations

EXAMPLE

Mass of spirit burner + lid before heating = 68.75g

Mass of spirit burner + lid after burning = 67.85g

Temperature of water in copper can before heating = 21.5 degrees C

Temperature of water in copper can after heating = 52.5 degrees C

It takes 4.2 joules of energy to heat up 1g of water by 1 degree C

So how much energy does methylated spirit give off per gram if there was 50g of water?

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## Continued

1: Find mass of meths burnt.

68.75 - 67.85 = 0.9g

2: Find temperature rise of the water due to heating.

52.5 - 21.5 = 31 degrees C

Therefore 0.9g of meths produces enough energy to heat up 50g of water by 31 degrees C

3: Find energy produced.

4.2 x 50 x 31 = 6510 joules.

0.9g of meths produces 6510 joules of energy, therefore 1g of meths produces:

6510/0.9 = 7233 Joules

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## Bond Energy Calculations

EXAMPLE

Calculate the energy change for the reaction: H2 + Cl2 --> 2HCl

Bond Energies:

H --- H: +436 kJ/mol

Cl --- Cl: +242 kJ/mol

H --- Cl: +431 kJ/mol

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## Continued

1: Find energy used up when breaking the bonds.

436 + 242 = 678 kJ

2: Find energy released when making the bonds.

2 x 431 = 862 kJ

3: Find overall energy change

862 - 678 =

= 184 kJ/mol released

This shows energy released, so it is actually -184 kJ/mol

This shows that this reaction is exothermic

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## Empirical Formula

EXAMPLE

0.4g of an organic hydrocarbon is burnt completely in oxygen. 1.1g of carbon dioxide and 0.9g of water are formed. What is the compounds empirical formula?

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## Continued

1: Find the mass of carbon in the compound.

1.1 x (12/44) = 0.3 g

2: Find the mass of hydrogen in the compound.

0.9 x (2/18) = 0.1 g

3: Divide the masses by the RAM

carbon : 0.3/12 = 0.025 mol

hydrogen : 0.1/1 = 0.1

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4: Divide by the smallest one to get the ratio of carbon to hydrogen.

0.025/0.025 = 1

0.1/0.025 = 4

= CH4

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