First a balanced reaction equation must be established:

→

Now if mass of aluminum = 145 g
the moles of aluminum = (MASS) ÷ (MOLAR MASS) = 145 g ÷ 30 g/mol
= 4.83 mols
Now the mole ratio of Al : O₂ based on the equation is 4 : 3
[
4Al +
3 O₂ → 2 Al₂O₃]
∴ if moles of Al = 4.83 moles
then moles of O₂ = (4.83 mol ÷ 4) × 3
=
3.63 mol (to 2 sig. fig.)
Thus it can be concluded that
3.63 moles of oxygen is needed to react completely with 145 g of aluminum.
The chemical equation without coefficients is:
Ca + CO2 + O2 --------> Ca CO3
You can balance that equation by trial an error.
This is the chemical equation balanced:
2Ca + 2CO2 + O2 --------> 2Ca CO3
Count the atoms on each side to check the balance
Atom Left side right side
Ca 2 2
C 2 2
O 2*2 + 2 = 6 2*3 = 6
Then those are the coefficients:
a0 = 2
a1 = 2
a2 = 1
a3 = 2
The answer is b. The sum of the individual gas pressure in the mixture
As the question tells you, you need to use the formula
% mass= mass of solute/ mass of solution x 100
mass solute= 30.0 g
mass of solution= 30.0 + 270.0= 300.0 g
% mass= 30.0/ 300.0 x 100= 10%
answer is B