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The amount of substance

- Scientists talk about the ‘amount of substance’
- What this means is how many atoms we have in a given amount of a substance.
- This is given the symbol nand the unit is the mole.

The Avagadro Constant

- The amount of substance is based on a standard count of atoms called the Avagadro constant- NA
- The Avagadro constant is the number of atoms per mole of the carbon-12 isotope.
NA= 6.02 x 1023 mol-1

Mass and mole

- The mass of one mole of any substance is simply the relative atomic mass in grams.
- One mole of H= 1 g
- One mole of C= 12 g
- One mole of Sn= 118.7 g

- Also works for compounds
- One mole of H2O= 18 g

Moles are big.

- How big is a mole? Some ideas to give you perspective:
- Stacked end to end, 1 mole of cereal boxes would stretch from the Sun to Pluto.
- One mole of marbles would cover the USA to a depth of four metres.
- One mole of red blood cells would be more than the total number of blood cells found in every human on earth.

- 7.5 million times.

Using the mole- Molar Mass

- Molar mass, M, is the mass per mole of a substance. Measured in grams per mole.
- You can find this by adding together all the relative atomic masses for each atom in a molecule.
- For oxygen: M(O2)= 16 + 16 = 32 g mol-1
- For methane: M(CH4)= 12 + (4 x 1) = 16 g mol-1

The amount of substance

- The amount of substance n, mass m, and molar mass M, are all linked.
m

M

n= number of moles, mol

M= molar mass, gmol-1

m= mass, g

n=

m

M

n

Examples

- In 48 g of C, n(C)=
- In 11 g of CO, n(CO)=
- In 4 g of CH4, n(CH4) =

Answers

- In 48 g of C, n(C) = 48.0/12.0 = 4.00 mol
- In 11 g of CO, n(CO) = 11.0/28 = 0.39 mol
- In 4 g of CH4, n(CH4) = 4.0/16 = 0.25 mol

More examples

- Calculating M and m is also possible:
- Calculate the amount in mol of:
- 8.00 g of SiO2
- 2.00 g of HCl
- 5.00 kg of Na

- Calculate the mass in g of:
- 4.00 mol of BeF2
- 6.00 mol of NaCl
- 0.25 mol of MgO

- Calculate the amount in mol of:

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