Question

10g of sulphur is burnt in 10g of oxygen to form 20g of sulphur dioxide. What mass SO2 is formed when 20g of sulphur reacts with 30g of oxygen.

Answer 1

Given:
S +    O2 ------------->  SO2
WHEN,
10g of S reacts with 10g of O2 we get 20 g of SO2
Given 20g of sulphur reacts with 30g of oxygen ----?
here we can see, sulphur  acts as limiting agent which limits production of SO2
Hence
20 g of Sulphur reacts with 20g of Oxygen to produce 40g of Sulphur dioxide.
and 10grams of oxygen is left unreacted.

Answer 2

The equation is as follows:

S  +  O2   ------------->   SO2

The mole ratio here is 1:1:1

That means 1 mole of oxygen reacts with 1 mole of sulphur to form 1 mole of sulphur dioxide.

The limiting reagent in this case is sulphur. 

Find the moles of sulphur:
moles = mass/molar mass  
           = 20/32 
           =0.625
The mole ratio is 1:1:1

That means that the mole of sulphur dioxide formed is also 0.625

Find the mass of SO2 formed:

mass = moles × molar mass
          = 0.625 × 64
          =  40g
The mass of SO2 formed thus is 40g