Question

2 C2H6 + 7 O2 yields 4 CO2 + 6 H2O

What volume of ethane is needed to react with 21L of oxygen? T and P have already been calculated

Answer 1

what are the values for P and T?

Answer 2

it doesn't say. it just says they have been calculated so I am assuming they must be constant?

Answer 3

yeah that means they just want to confuse you

Answer 4

is there anymore info? like if the reaction was carried out under room temp and pressure or standard temp and pressure

Answer 5

i think most probably its STP

Answer 6

what do u think?

Answer 7

ye i think it must be STP, but this was found in the law of combining volumes section.

Answer 8

anyways
the formula u use here is
\[volume = no. of moles \times STP\]

Answer 9

are u familiar with this formula?

Answer 10

not really. so should i convert the 21:L O2 to ethane mole and multiply by the STP?

Answer 11

nah first u find mole of 21 L of O2

Answer 12

ok

Answer 13

so convert L to grams then find moles of O2 then convert to ethane then multiply by stp?

Answer 14

no of mole= volume/STP
= 21/22.4= 0.9375 mole

Answer 15

nooo forget about grams
u fdon't even need grams

Answer 16

don't*

Answer 17

hmmmm ok, but the answer is 6L

Answer 18

yeah wait lol

Answer 19

so now u know the mole of O2 and now look at the ratio between O2 and C2H6

Answer 20

its 2:7 right?

Answer 21

so if
7---- 0.9375 moles
2---- ?
so do cross multiplication and find the mole of C2H6

Answer 22

are u getting it?

Answer 23

anyways do this
you'll get the mole
since you do it in calculator, do this
0.9375*2/7= .....mole and do not round off and straightaway multiply it by 22.4 = and u'll get 6 L

Answer 24

btw here is the link
for proving that its STP
http://chemistry.tutorvista.com/inorganic-chemistry/law-of-combining-volumes.html

Answer 25

ok thanks

Answer 26

did u get it though?

Answer 27

yes. what I did not know was the #moles=volume/STP

Answer 28

yeah good then :) happy to help :D