The Mole -How do we determine the amount of a substance? -Calculating molar mass

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 n and the unit is the mole.

The Avagadro Constant The amount of substance is based on a standard count of atoms called the Avagadro constant- N A The Avagadro constant is the number of atoms per mole of the carbon-12 isotope. N A = 6.02 x 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= g Also works for compounds – One mole of H 2 O= 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(O 2 )= = 32 g mol -1 – For methane: M(CH 4 )= 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 M m n= n= number of moles, mol M= molar mass, g mol -1 m= mass, g

Examples In 48 g of C, n(C)= In 11 g of CO, n(CO)= In 4 g of CH 4, n(CH 4 ) =

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 CH 4, n(CH 4 ) = 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 SiO g of HCl 5.00 kg of Na – Calculate the mass in g of: 4.00 mol of BeF mol of NaCl 0.25 mol of MgO