Question

What is the molecular formula of a compound made of 25.9% N and 74.1% O?

Answer 1

D.

If we take a sample of 100g of the given compound, we should find that it contains 74.1g of oxygen and 25.9g of nitrogen. The molar masses of oxygen and nitrogen are approximately 16.0g/mol and 14.0g/mol respectively. To find the appropriate number of moles of oxygen and nitrogen found in one mole of the given compound, we divide the mass of the sample by the molar mass of each substance.

For oxygen: 74.1g/(16.0g/mol) = 4.63125 mol (round up to 5 mol)

For nitrogen: 25.9g/(14.0g/mol) = 1.85 mol (round up to 2 mol)

Therefore one mole of this compound contains 2 moles of nitrogen and 5 moles of oxygen.

Answer 2

74.1% + 25.9% = 100%

we assume 100g of sample

100%-----------100g
25.9% N ------------x
x 25.9g N

100g - 25.9g = 74.1g O

mass of N = 14g
mass of O = 16g

1 mole of N ------------ 14g
x moles of N -------------- 25.9g
x = 1.85 moles of N

1 moles of O -------------- 16g
x moles of O ----------------- 74.1g
x = 4.6 moles of O

N : O = 1.85 : 4.6
we share by the smallest value
N : O = 1.85 : 4.6  ||:1.85
N : O = 1 : 2.4 ≈ 1 : 2
NO₂
answer: B