Zn + S -> ZnS
Note that I didn't write S8, S8 is only one of the forms of sulphur and it doesn't matter for the calculations anyway.
Atomics weights of Zn and S are 65.41 and 32.07 resp.
So, 25g of Zinc will react with 25*32.07/65.41 = 12.2573 g of Sulphur.
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One mole Al2O3 weights 2*26.98 + 3*16 = 101.96 g and contains 2*26.98 = 53.96g Al.
Dissociation of 100g Al2O3 will give 100*53.96/101.96 = 52.9227 g Al. The remainder (100-52.9227=47.0773g) is the Oxygen of course which is also found by 100*3*16/101.96= 47.0773.
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Propane C3H8 burns following the equation:
C3H8 + 5 O2 -> 3 C02 + 4 H2O
Propane's weight is 3*12 + 8*1 = 44, O2's weight is 2*16 = 32
One mole propane needs five moles oxygen to burn, that is 5*32/44 = 3.6364 times its weight. So, 10 g propane needs 36.364 g of oxygen to burn.
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