Quantitative chemistry

Question 1

how do you find the number of moles?

Answer 1

moles = mass/Ar or Mr

Question 2

how do you find the molecular formula?

Answer 2

actual Mr/empirical Mr
say 28/14 = 2
so mf = 2 x ef.

Question 3

how do you find the mass of CuO in this equation
when Cu = 6.4g
2Cu + O2 -> 2CuO

Answer 3

2Cu + O2 -> 2CuO
1. find moles of given substance
moles of Cu = 6.4/63.5 = 0.101 moles
2. use ratio to find wanted subs. moles
moles of Cu = moles of CuO = 0.101 moles
3. calculate mass
mass = moles x Mr/Ar (in this case Mr)
mass = 0.101 x (63.5 + 16)
mass of CuO = 8.03g

Question 4

how do you find the % mass of Na in Na2CO3?

Answer 4

% Na = Ar x no. of atoms in molecule/Mr all this x 100
= 23 x 2 / (2 x 23) + 12 + (3 x 16) all x 100
= 43.4%

Question 5

how do you find the empirical formula of iron oxide?

Fe = 44.8g and O = 19.2g

Answer 5

Fe O
mass/Ar 44.8/56 19.2/16
/smallest no. 0.8/0.8 1.2/0.8
sometimes x
to get integer 1 x 2 1.5 x 2
2 : 3
E.f = Fe2O3

Question 6

how do you find the Mr?

Answer 6

add all Ar of each element
e.g MgCl2
Mr = 24 + (2 x 35.5)
= 95