Chemistry - The Mole Concept

bonkysleuth SG

Need a little help with this. I don't really understand the question and what it is asking for.

A 1.50g sample of hydrocarbon undergoes complete combustion to produce 4.40g of CO2 ad 3.60g of H2O. What is the empirical formula of this compound? In addition, its molercular weight has been determined to be about 16. What isthe molecular formula?

Hint : hydrocarbon contains only H & C. Find mass of carbon and hydrogen first.

It seems a little strange to me. What's the compound they are talking about? Is it the combination of CO2 and H2O? But how do you get the mass of carbon and hydrogen? through simultaneous equation? That is just a quaint suggestion of mine.

Please help as I haven't a single idea how to get the mass of carbon and hydrogen out... or perhaps the empirical formula. I should be able to tackle the rest on my own. THANKS!

jiaxing2 SG

hydrocarbon undergo complete combustion to form CO2 and H20

general formula for the combustion goes like this:

CxHy + (x+y/4) O2 -----------> XCO2 + (y/2) H20

notice Rhs balance wif Lhs

we know that the products consist of 4.4g CO2....Mr (CO2) = 44

thus x = 0.10

Mr (H2O) = 18

y/2 = 3.6/18

y = 0.4

ratio of x:y = 1 : 4........empirical formula and molecular coincide...CH4 ( Mr = 16 )

UltimaOnline SG

First, write an equation for the combustion of CxHy, and work out the coefficients of the remaining terms (O2, CO2 and H2O) in x and y. It'll only take you 10 seconds to do so (do NOT memorize the coefficients! I always tell my students to understand and work it out yourself instead of blindly memorizing! You already got so much to memorize, why torture yourself further?)

You will then, of course, obtain (see my stickied post on balancing equations if you are unable to obtain this balanced equation)

CxHy + (x + y/4) O2 --> x CO2 + y/2 H2O

Since you're given the sample mass AND molar mass (aka relative molecular mass) of the hydrocarbon, work out the no. of moles of the hydrocarbon.

Hence, you can obtain :

(i) no. of moles of CO2 produced in combustion, based on stoichiometry or the balanced equation.

(ii) no. of moles of CO2 produced in combustion, based on experimental data (you're given sample mass).

(iii) no. of moles of H2O produced in combustion, based on stoichiometry or the balanced equation.

(iv) no. of moles of H2O produced in combustion, based on experimental data (you're given sample mass).

Simply equate (i) and (ii), and you will obtain the value of x.

Simply equate (iii) and (iv), and you will obtain the value of y.

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(Edited : jiaxing2 has worked it out correctly.)

UltimaOnline SG

Here are some 'O' level questions (but 'A' level students should also try them for your own revision and practice too!), related questions (variants of bonkysleuth's question) for students' practice and enjoyment :

A sample of 0.43g of an organic compound containing only carbon, hydrogen and oxygen, was burnt in excess oxygen. The combustion produced 1.10g of carbon dioxide and 0.45g of water.

a) Calculate the empirical formula of the compound.

b) Given that this compound has a relative molecular mass of 250g, deduce its molecular formula.

Ans : C15 H30 O3

2) 20cm³ of a gaseous hydrocarbon was mixed with 100cm³ of oxygen so that the hydrocarbon was completely burnt. The volume of gas remaining at the end of the combustion was 70cm³. After passing through soda lime, the volume was reduced to 10cm³. All gases were measured at r.t.p. Determine the formula of the hydrocarbon.

Ans : C3 H6

3) A mixture of MgSO₄.7H₂O and CuSO₄.5H₂O is heated until a mixture of the anhydrous salts, is obtained. If 5.0g of the hydrated mixture when heated gives 3.0g of the anhydrous salts, calculate the % by mass of CuSO₄.5H₂O in the initial hydrated mixture.

Ans : 73.9%

4) (Ok, this isn't as closely related to BonkySleuth's qn as the above 3, but is nonetheless a fun classic that my students enjoy very much, so here goes)

When Fe and Fe³⁺ are mixed together, a reaction occurs in which Fe²⁺ is produced. What is the ratio of Fe to Fe³⁺ required to produce equal moles of Fe²⁺ and Fe³⁺ when the reaction is complete?

Ans : (For fun's sake, I'll leave it to you to work out the answer. You can post your answer to check with me if you like).

Everyone Loves Chemistry!

bonkysleuth SG

You will then, of course, obtain (see my stickied post on balancing equations if you are unable to obtain this balanced equation)

CxHy + (x + y/4) O2 --> x CO2 + y/2 H2O

Since you're given the sample mass AND molar mass (aka relative molecular mass) of the hydrocarbon, work out the no. of moles of the hydrocarbon.

Hence, you can obtain :

(i) no. of moles of CO2 produced in combustion, based on stoichiometry or the balanced equation.

(ii) no. of moles of CO2 produced in combustion, based on experimental data (you're given sample mass).

(iii) no. of moles of H2O produced in combustion, based on stoichiometry or the balanced equation.

(iv) no. of moles of H2O produced in combustion, based on experimental data (you're given sample mass).

Simply equate (i) and (ii), and you will obtain the value of x.

Simply equate (iii) and (iv), and you will obtain the value of y.

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(Edited : jiaxing2 has worked it out correctly.)

Thanks so much! I have one question. You just have to tell me what the basis of comparison is. 0.3kg of sodium hydroxide is dissolved in 5 litres of water. It is used as a titrant with 20.0cm3 of sulphuric acid of a concentration of 0.5mol/dm3. what is the volume of sodium hydroxide solution that is required to neutralize all the sulphuric acid?

Right now, I have worked out the concentration of NaOH, which is mass of solute/vol of solution

you get 60g/dm3. so to find the volume of added NaOH, do you do so in terms of the concentration or any other stuff? if it is concentration, it doesn't make sense because you cant change a solution's concentration. However, it seems like there isn't any other way to compare other than concentration. I find the concentration, changed them to get the no. of moles involved in the reaction for each reactant and compare using mole ratio. can it be done this way?

Please reply ASAP. urgent. thanks.

FrozernFire SG

find the concentration of the NaOH solution in mol dm-3. find the no. of moles of NaOH needed to neutralise the H2SO4, then find the volume of NaOH solution needed

bonkysleuth SG

find the concentration of the NaOH solution in mol dm-3. find the no. of moles of NaOH needed to neutralise the H2SO4, then find the volume of NaOH solution needed

How do i get the no. of moles of NaOH needed to neutralise H2SO4? my idea is that you write down a balanced chemical equation and compare the mole ratio of NaOH and H2SO4.(because neutralisation takes place with the reaction of the 2 products) is my assumption correct or wrong? please rectify if it's incorrect.

tut4nkh4m3n SG

chem needs practice... like a lot a lot a lot... sian.

UltimaOnline SG

0.3kg of sodium hydroxide is dissolved in 5 litres of water. It is used as a titrant with 20.0cm3 of sulphuric acid of a concentration of 0.5mol/dm3. what is the volume of sodium hydroxide solution that is required to neutralize all the sulphuric acid?

Solution :

1) Find the molarity of OH-. (no. of moles / volume of soln).

3) Find the no. of moles of protons H+ in 20cm3 of 0.5M of H2SO4.

4) Hence state the no. of moles of OH- required for titration.

5) Let the required volume of NaOH be v (algebraic variable) dm3.

6) Solve for v in the equation : No. of moles of OH- required = (molarity of OH-) x (v dm3).

UltimaOnline SG

FrozernFire is correct.

tut4nkh4m3n, trust me, Chemistry is the most fun of all subjects to take at 'A' levels.

tut4nkh4m3n SG

i agree that it's one of the most fun subjs, but you must put in quite a lot of effort indeed

UltimaOnline SG

i agree that it's one of the most fun subjs, but you must put in quite a lot of effort indeed

Well, every subject must put in a lot of effort at 'A' levels and beyond. But when you enjoy a subject very much, it becomes effortless to score distinctions.

Thanks so much!

Oh, you're welcome, Bonkysleuth.