Question

6. Challenge yourself: Suppose you wanted to make 100 grams of water.
A.
What is the molar mass of water (H2O)?
B.
How many moles of water are in 100 grams?
C. How many moles of hydrogen will you need?
D. How many moles of oxygen will you need?
E. How many grams of hydrogen and oxygen will you need?
F.
Is your answer reasonable? Why or why not?

Answer 1

Based on the mass of water to be produced;

• Molar mass of water = 18 g/mol
• moles of water = 5.55 moles
• Moles of hydrogen needed = 5.55 moles
• Moles of oxygen needed = 2.775 moles
• Mass of hydrogen needed = 11.11 g
• Mass of oxygen needed = 88.8 g

What is the molar mass of water?

A water molecule contains two moles of hydrogen atom and 1 mole of oxygen atom.

H = 1; O = 16

Molar mass of water = 18 g/mol

Equation of reaction: 2H₂ + O₂ ---> 2 H₂O

Moles of water in 100 g

Moles = mass /molar mass

moles of water = 100/18

moles of water = 5.55 moles

Moles of hydrogen needed

1 mole of water contains 2 moles of hydrogen atom

There are 2 atoms in one molecule of hydrogen gas

Moles of hydrogen needed = 2 * 5.55/2

Moles of hydrogen needed = 5.55 moles

Moles of oxygen needed

1 mole water contains 1 mole oxygen atom

There are two moles of atoms in 1 mole of oxygen gas

Moles of oxygen needed = 1 * 5.55/2

Moles of oxygen needed = 2.775 moles

Grams of hydrogen and oxygen needed

1 mole hydrogen gas = 2.0 g

1 mole oxygen gas = 32.0 g

Mass of hydrogen needed = 5.55 * 2 = 11.11 g

Mass of oxygen needed = 2.775 * 32 = 88.8 g

total mass = 88.8 +11..11 = =99.91 g

The answer is reasonable because the total mass of reactants is approximately equal to mass of product required.

Therefore, based on the mass of water to be produced;

• Molar mass of water = 18 g/mol
• moles of water = 5.55 moles
• Moles of hydrogen needed = 5.55 moles
• Moles of oxygen needed = 2.775 moles
• Mass of hydrogen needed = 11.11 g
• Mass of oxygen needed = 88.8 g

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