Q&A What

What is the total mass of oxygen that must react with 8 grams of hydrogen to form 72 grams of water

A. 8.0g

B. 16.0g

C. 36.0g

D. 72.0g

QUICK ANSWER…

DETAILS…

To resolve this, we must write out the balanced equation thus: 2H₂ + O₂→ 2H₂O

From this balanced equation we can infer that:

2 moles of hydrogen is required to form 2 moles of water

Now, we have to determine how many moles of hydrogen is equivalent to a mass of 8g

2g of H₂ = 1 mole, therefore,

8g of H₂ = x mole.

Cross multiplying to solve for x, we have:

2x = 8

x = 8/2 = 4moles

this implies that 4 moles of hydrogen is present.

Now, if from the balanced equation,

2 moles of H₂ forms 2 moles of H₂O, then

4 moles of H₂ will form Y moles of H₂O

Cross multiplying to find Y, we have:

2Y = 4(2) = 8

Y = 8/2 = 4

This implies that the number of moles of water formed = 4 moles.

Now converting these 4 moles of water to grams, we have:

Molar mass of H₂O = 2 + 16 = 18g

This implies that 1 mole of H₂O = 18g, therefore,

4 moles will be = 4 × 18 = 72g

SHORT-CUT

There’s a short cut solution,

using the balanced equation,

2H₂ + O₂→ 2H₂O

mass of 2H₂ = 2(2) = 4g

mass of 2H₂O = 2(2+16) = 2(18) = 36g

this implies that:

4g of hydrogen gas produces 36g of water, therefore,

8g of hydrogen gas will produce X

cross multiplying to solve for X, we have:

X(4) = 8(36)

X = (8 × 36)/4 = 72g

Now for the right answer to the above question:

Option A is incorrect.
Option B is incorrect.
C is incorrect.
D is the correct answer.

KEY-POINTS…

You may please note these/this:

For accuracy and clarity purposes, you must write out the balanced chemical equation of questions like this.
The importance of writing out the balanced equation is to properly understand the mole ratios involved, and to make your assumptions properly.
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/ culled from 2020 JAMB-UTME chemistry past question 32 /

Therefore, 8 g of H2 is 4 moles. Therefore you will form 4 moles of water. The molar mass of water is 18 g/mol so you will form 4 x 18 = 72 g of water.

What is the total mass of water formed when 8 grams of hydrogen react with 64 grams of oxygen?, 1 g H2 reacts with= 8 g Oxygen. So 64 g of oxygen reacts with= 8 g H2. So (64+8)=72 g of water will be formed.

Furthermore, What is the total mass of water formed when 8 grams of hydrogen reacts completely with?, Answer: The total mass in grams of water formed when 8.00 grams of hydrogen reacts completely with excess oxygen is 71.5 g.

Finally, What weight of water is formed when 8 grams hydrogen and 8 g oxygen react together?, The molar mass of hydrogen is 2, oxygen is 32 and water is 18. Hence, with 8 grams of both hydrogen and oxygen, the mass of water produced is 9 gram.

Frequently Asked Question:

The explanation is: The chemical equation of water formation is 2H2 + O2 → 2H2O. Though we have 8g of hydrogen, here oxygen is the limiting reagent. So the only 4g of hydrogen can be used to produce water i.e. 36g of water. That is 2 moles.