# Calculations in chemistry by LisaB1982

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```									Understanding Chemical Reactions
Lesson: Calculations in Chemistry

The Mole

The mole
• The number of atoms in 1 gram of hydrogen is called a mole.
• 1 gram of hydrogen contains approx. 6 X 1023 atoms. • 600, 000, 000, 000, 000, 000, 000, 000 atoms – rather a lot!

The mole
• This number is called Avogadro’s constant.

The mole
• One mole of any substance always contains the same number of particles. • The mass of one mole of a substance is called the Relative Atomic Mass (RAM).

R.A.M.

• There are huge numbers of atoms even in test-tube reactions. • So it is easier to talk about moles than the actual numbers involved.

Learn this equation!!!!

Moles of atoms

=

mass R.A.M.

• It’s like cashing in 1p coins at the bank. • If you take a thousand 1 p coins to your bank, does the cashier count out each coin? • The coins are weighed out on scales. The scales ‘know’ the mass of one hundred 1p coins, and tell the cashier how many pounds (£s) are on the scales.

Example
• How many moles of atoms are there in 2.4 g of carbon? • Moles of atoms = mass / RAM = 2.4/12 = 0.2 moles

Practice
Try these yourself! How many moles are in: 1. 2g of hydrogen 2. 36g of carbon 3. 160g of oxygen 4. 1.4g of nitrogen 5. 0.19g of flourine? (R.A.M.s H = 1, C = 12, O = 16, N = 14, F = 19)

1. 2 moles 3. 10 moles 5. 0.01 moles 2. 3 moles 4. 0.1 moles

Relative Formula Mass
• Once you have a list of relative atomic masses, (which are shown on the Periodic Table), you can work out the relative mass of any molecule. • The relative atomic of CO2 • The RAM of C is 12 • The RAM of O is 16

Relative Formula Mass
• So for CO2 we have: • 1 carbon = 1 X 12 = 12 • 2 oxygen = 2 X 16 = +32 • Therefore the relative formula mass of carbon dioxide is 44.

Example
• What is the relative formula (or molecular mass) of aluminium sulphate, Al2(SO4)3 • (R.A.M.s: Al = 27, S =32, O =16)

Example
• What is the relative formula (or molecular mass) of aluminium sulphate, Al2(SO4)3 • (R.A.M.s: Al = 27, S =32, O =16) • 342 g !

Questions
• Answer the questions off p175 of the Chemistry text.

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