[angstyplankton]

I'm currently studying chemical calculations, inclusive of the Mole concept and all. Here's an overview of what I've learnt:

• Relative atomic mass (Ar)
• Relative molecular mass (Mr)
• Avogadro's constant = 6 * 10^23
• Number of moles = Mass of reactant/Relative mass
• Number of moles = Number of particles/Avogadro's constant
• Avogadro's law (1 mol takes up 24 dm^3 at r.t.p)
• Number of moles = Volume/24dm^3/mol
• Mass/mole concentration = Mass/Mole/Volume

So that's what I've revised. But I have two questions which I need explanation for:

1. Observe the following reaction:

2 C2H6 (g) + 7 O2 (g) --> 4 CO2 (g) + 6 H2O (l)

If 378 grams of water was produced in this reaction, find:
(a) mass of carbon dioxide produced,
(b) mass of ethane (C2H6) produced,
(c) mass of oxygen gas required.

The answers for this question are:
(a) 616 g
(b) 210 g
(c) 784 g


2. Observe the following reaction:

Na2CO3 (aq) + 2 HCl (aq) --> 2 NaCl (aq) + CO2 (g) + H2O (l)

In the above reaction, 40 cm^3 of 0.200 mol/dm^3 HCl was used to react with Na2CO3:
(a) find the number of moles of HCl used,
(b) find the number of moles of Na2CO3 used,
(c) find the volume of Na2CO3 required, given that its concentration is 0.150 mol/dm^3,
(d) find the mass of water formed.

The answers for the questions are:
(a) 0.00800 mol
(b) 0.00400 mol
(c) 0.0250 dm^3
(d) 0.0720 g

If you can solve the questions, can you provide an explanation?

Thanks in advance!

[Schrödinger]

I'll give you hints for the first question. Maybe that will help you understand better.

Observe the given reaction.
To put it in words, the reaction says that every 2 moles of ethane requires 7 moles of oxygen to produce 4 moles of carbon di oxide and 6 moles of water.

You know the molar mass of each of these substances.
CO2 = 44g/mol
H2O = 18g/mol
O2=32g/mol
C2H6=30g/mol

Find the mass of the compounds assuming the number of moles to be as per the given equation. Then find the ratio of the compounds (mass/mass ratio)

So now you know the ratio in which the reactants and products are formed. From here it should be easy.

[angstyplankton]

I'll give you hints for the first question. Maybe that will help you understand better.

Observe the given reaction.
To put it in words, the reaction says that every 2 moles of ethane requires 7 moles of oxygen to produce 4 moles of carbon di oxide and 6 moles of water.

You know the molar mass of each of these substances.
CO2 = 44g/mol
H2O = 18g/mol
O2=32g/mol
C2H6=30g/mol

Find the mass of the compounds assuming the number of moles to be as per the given equation. Then find the ratio of the compounds (mass/mass ratio)

So now you know the ratio in which the reactants and products are formed. From here it should be easy.

I understand where you are hitting at up to the second part, "Find the mass of the compounds assuming the number of moles to be as per the given equation." – what do you mean by "as per the given equation?"

If 378 g of H2O was produced, and there is 6 mol of H2O, is this what I should do:

Number of moles * Molar mass = Relative mass

6 mol * 18 g/mol = 108 g (Relative molecular mass)

Mass of reactant / Relative mass = Number of moles

378 / 108 = 3.5 mol

Therefore, there should be 3.5 mol of H2O for 378 g of H2O to be produced, is that right?

I'm clueless after this.

Thanks for the enlightening hints, but I'm still a little confused.

[Schrödinger]

Yes. You got the number of mole right. It is 3.5

Now, from the given equation, you seen that for every 6 moles of H2O produced, there are 4 moles of CO2 produced. Using this data, find out how many moles of CO2 will be produced given then there are 3.5 moles of H2O. Then convert the moles of CO2 obtained into mass in grams...