mole concept & stoichiometry

Question 1

percentage composition by mass in compound (e.g. of X)

Answer 1

(sum of Ar of all atoms of element X / Mr of compound) x 100%

Question 2

mass of element X in sample

Answer 2

(sum of Ar of all atoms of element X in sample / Mr of compound) x mass of sample

Question 3

empirical formula steps

Answer 3

1) mass ratio of elements involved
2) convert mass ratio into mole ratio (divide each mass by molar mass of respective element)
3) determine smallest whole number for mole ratio to be obtained

Question 4

number of particles

Answer 4

amount (mol) x avogrado constant (6.02 x 10^23)

Question 5

mass (g)

Answer 5

amount (mol) x molar mass (g/mol)

concentration (g/dm3) x volume (dm3)

Question 6

volume of gas (dm3)

Answer 6

amount (mol) x stp OR rtp (22.7 OR 24.0 dm3/mol)

Question 7

amount of solute (mol)

Answer 7

concentration (mol/dm3) x volume (dm3)

Question 8

1 cm3 in dm3

Answer 8

0.001dm3

(1 dm3 = 1000cm3)

Question 9

volume of gas is directly proportional to?

Answer 9

number of moles of the gas

so mole ratio = volume ratio

Question 10

percentage yield

Answer 10

actual/theoretical x 100%

(in terms of mass/volume/amount)

identify actual yield first

Question 11

percentage purity

Answer 11

mass of pure/mass of impure x 100%

mass of impure is given; cannot be calculated, cannot change to moles