Help me to answer these questions

1. Nov 6, 2005

almirza

1)
10 cm^3 of a gas G were mixed with 100 cm^3 (an axcess) of oxygen in a sealed container at 20 C and one atmosphere pressure . The gas was then ignited electrically and burned completely in the oxygen. After the reaction the volume of the gas mixture measured at the the same temperature & pressure , was found to be 105 cm^3. Which of the following was G?
A CH4
B H2
c CO
D C2H4

and how??????

2)
10 cm^3 of a gaseous hydrocarbon needed 30 cm^3 of oxygen for complete combustion and 20 cm^3 of CO2 were produced . What is the molecular formula of the hydrocarbon and how?
A CH4
B C2H2
C C2H4
D C2H6

3)
In the thermit reaction :
2Al + Cr2O3 produce 2Cr+ Al2O3
Calculate the percentage yield when 180 g of chromium are obtained from a reaction between 100 g of aluminium and 400 g of chromium
.
4)
A sample of hydrated compound was analysed and found to contain 2.10g of cobalt , 1.14g of sulphur , 2.28g of oxygen and 4.50g of water. Calculate the emperical formula
.
5)
A 10.0g of a compound contained 3.91g of carbon , 0.87g of hydrogen and the reminder is oxygen. Calculate the emperical formula of the compound
.

[ these questions are not difficult but there are some small problem make me not sure ]

2. Nov 6, 2005