Reacting Masses - Worked Example

Step by step video showing you how to do the reacting masses calculations. In these questions, you are given the mass of the limiting reactant and asked to calculate the mass of a product, using the balanced chemical equation.
If you are practising, here are some questions for you to try. Show your full working out and see if you arrive at the correct answer:
1. 56 grams of water (H2O) react with sodium (Na) to produce sodium hydroxide (NaOH) and hydrogen gas (H2). Calculate the mass of hydrogen produced. The balanced chemical equation for the reaction is: 2Na + 2H2O  2NaOH + H2
Answer: 3.12 grams
2. Calculate the mass of potassium oxide (K2O) produced from 126 grams of potassium (K) reacting with oxygen (O2) in excess. The balanced chemical equation for the reaction is: 4K + O2  2K2O
Answer: 152 g
3. 34 grams of magnesium (Mg) react with chlorine (Cl2), which is in excess. Calculate the mass of magnesium chloride (MgCl2) produced. The balanced chemical equation for the reaction is:
Mg + Cl2  MgCl2
Answer: 140 grams
4. Excess sulfuric acid (H2SO4) reacts with 23 grams of copper oxide (CuO). Calculate the mass of copper sulfate (CuSO4) that would form. The balanced chemical equation for the reaction is:
CuO (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
Answer: 46.1 grams

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[@sujatadasroy4269]

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[@insync6684]

Your techniques is very good but instead of using moles we can calculate like this, this is a easy method.
first -
find the relative atomic mass of 4Fe
so, 4 x 56 = 224g
secondly find the atomic mass of iron oxide
iron = (56 x 2)
= 112
112 x 2 (because there are 2 moles in iron)*
=224g
Oxygen = (16 x 3)
=48
48 x 2 ( because there are 2 mole in oxygen)
= 96g
Now we add the masses 224 + 96
= 320g
We know that
4fe 2fe(2)O(3)
224g - 320g
45g - ?
224 g produce 320g grams of iron oxide, how much does 45g produce?
45 x 320
-------------- = 64.28g = 64.23g ( 3 sgf)
224
If you guys found this easy try this one or try another one.
your choice.

[@otaku1615]

Thx alott I just got a question to determine the molar mass of 3O2 for example do we multiply the molar mass by 2×3 or just by 2 or just by 3?

[@insync6684]

@@otaku1615 First. What is the molecular mass of oxygen ? 16. The question is 3O2, so your first times by 2. So 16 x 2 = 36. THEN times by 3 (the mole)
. 36 x 2 = 72. The total mass of 3O2 is 72.

[@otaku1615]

@@insync6684 oh okay so for another example if we got 7O2 we first times the molar mass of O which is 16 by 2 then by 7

[@insync6684]

@@otaku1615 yes

[@otaku1615]

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[@henrycarr7983]

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Really helpful. Thank you

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[@RhondaGomaveh-pf8bo]

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[@watashiwaaziz7252]

Since you have the mol ratio, and the no. Of mol for iron, you could just cross multiply, ( quicker method k

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[@ksumar]

I think the solution can be simplified much easily without the mole concept:
From the equation we know that:
4Fe + 3O2 ---> 2Fe2O3
(56x4=224) of Iron produces 320g of Fe2O3
(twice as much as mole of Fe=160g)
Therefore, if
224g Fe------> 320g Fe2O3
45g------> give would produce:
45 x 320/224 = 64.3g of Iron III Oxide!

[@samanthavirchow6263]

I'm confused how you found the ratio of 4:2?

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[@cocopeppi7454]

why dont we have to worry about the oxygen in 1:54 ?

[@loredfolk1069]

why do we not factor in the o2?

[@elestrello6564]

we are looking for the iron thats why.

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