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really hard exam question please help me!!

Isooctane reacts with an excess of chlorine to form mixture of chlorinated compounds, one of these compunds has 24.6%Carbon 2.56Hydrogen 72.8%Chlorine, calculate molecular formula

so I calculated the empirical and got 1:1.25:1 and the markscheme says multiply that by 4 to give the empricial formula C4H5CL4
but I don't understand why u multiply by 2 to give the molecular formula C8H10CLl8?why ?? how do u get the molecular formula? normally they give the rmm of the molecule and then u have to calculate the empirical mass which for C4H5C4 is 195?
Original post by usernamenew
Isooctane reacts with an excess of chlorine to form mixture of chlorinated compounds, one of these compunds has 24.6%Carbon 2.56Hydrogen 72.8%Chlorine, calculate molecular formula

so I calculated the empirical and got 1:1.25:1 and the markscheme says multiply that by 4 to give the empricial formula C4H5CL4
but I don't understand why u multiply by 2 to give the molecular formula C8H10CLl8?why ?? how do u get the molecular formula? normally they give the rmm of the molecule and then u have to calculate the empirical mass which for C4H5C4 is 195?


You have not given us all of the required information (like the actual question)
Reply 2
Original post by charco
You have not given us all of the required information (like the actual question)


Capture.PNG
Isooctane has the formula C8H18
Monochloroisooctane has the formula C8H17Cl

There are two isotopic products one with a Mr of 148 and the other with Mr = 150

The ratio of the two products is given as 1.5 to 1.0 so the average Mr = [(3 x 148)+(2 x 150)]/5 = 148.8

24.6% carbon
2.56% hydrogen
(100 - 24.6-2.56)% chlorine

2.05 C; 2.56 H; 2.05 Cl

C; 1.25H; Cl

Multiply through by 4 to remove fraction.

C4H5Cl4

BUT you are told that it's a chlorinated octane, i.e it has eight carbon atoms, so you must MTB 2 to get eight C atoms

Molecular formula C8H10Cl8
Reply 4
Original post by charco
Isooctane has the formula C8H18
Monochloroisooctane has the formula C8H17Cl

There are two isotopic products one with a Mr of 148 and the other with Mr = 150

The ratio of the two products is given as 1.5 to 1.0 so the average Mr = [(3 x 148)+(2 x 150)]/5 = 148.8

24.6% carbon
2.56% hydrogen
(100 - 24.6-2.56)% chlorine

2.05 C; 2.56 H; 2.05 Cl

C; 1.25H; Cl

Multiply through by 4 to remove fraction.

C4H5Cl4

BUT you are told that it's a chlorinated octane, i.e it has eight carbon atoms, so you must MTB 2 to get eight C atoms

Molecular formula C8H10Cl8


thank u so much for explaining
Reply 5
where does it say its an isooctane chlorinated compound
Reply 6
Original post by usernamenew
Isooctane reacts with an excess of chlorine to form mixture of chlorinated compounds, one of these compunds has 24.6%Carbon 2.56Hydrogen 72.8%Chlorine, calculate molecular formula

so I calculated the empirical and got 1:1.25:1 and the markscheme says multiply that by 4 to give the empricial formula C4H5CL4
but I don't understand why u multiply by 2 to give the molecular formula C8H10CLl8?why ?? how do u get the molecular formula? normally they give the rmm of the molecule and then u have to calculate the empirical mass which for C4H5C4 is 195?
Calculate the above
Original post by Annaleanne
Calculate the above
Handy tip: Check the date on the OP before replying ....

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