he nitrates of calcium, strontium or barium are often added to firework mixtures to produce red or green flames. The equation for the decomposition of one such mixture is as follows. Sr(NO3) 2(s) + 3C(s) ⎯→ SrO(s) + N2(g) + 2CO2(g) + CO(g) Calculate the volume of gas given off (measured at room temperature and pressure) when a 10.0 g sample of this mixture decomposes. [Mr : Sr(NO3) 2, 211.6] .

Here's what I did :

10/211.6+32 = 0.0404

4:0.0404

1: x

i got x to be 0.0101 (these are the moles of strontium nitrate)

and then i used mole ratio to find the moles of the individual gases and i multiplied the moles of each individual gas by 24 to get the volume but this method seems to be incorrect as in the marking scheme they've done the following :

0.0404 x 4 = 0.162 moles

0.162 x 24 = 3.88 dm^3

Can someone please help