isaac physics a level physics help

hi i need help with my isaac physics board on ideal gases
g2.3
How many molecules of gas do you need in a 100cm ^3
cylinder to exert a pressure of,1.0×10 ^8
Pa at a temperature of 800 ∘ C? Give your answer to 2 significant figures.
2.7
A water fire extinguisher contains about four litres of air at 10 ^7 Pa and 20 ∘ C. When the extinguisher is used, this gas forces the water out. Calculate the pressure when the volume has increased to ten litres and the temperature has dropped to about 3.0 ∘ C. Give your answer to 2 significant figures.
thx

Reply 1

Joinedup

The data and formula booklet you're given in the A level exam (and is also downloadable from the exam board website) often contains useful formulae
gas laws.JPG

and constants

Reply 2

Tsuyu

For the first question, you can use the formula pV=NkT where p is the pressure, V is the volume, N is the number of molecules, k is the Boltzmann constant (1.38x10^-23) and T is the temperature in kelvin. Just be careful with the units and convert cm^3 to m^3 for the volume.
For the second one, pV/T is constant, so (p1V1)/T1=(p2V2)/T2 (note that the 1 and 2 here just means before and after). You have all the values for this equation to solve for p2. Again, don't forget to change the units. Hope this helps.