We know that the sea water contains $(65xx10^-3"g of bromide ions")/L $
If you want $Br_2$ the following reaction takes place
$Br^-) + Br^-) = Br_2 + 2e^-$
Well this reaction cannot take place but this can be a half reaction
But this reaction can take place
$2"HBr" +"H"_2"SO"_4 rightleftharpoons "Br"_2 + "SO"_2 + 2"H"_2"O"$
So this like
1mol of Br- reacts with 1mol of Br- to form 1 mol of $Br_2$
or 2mol of $"Br"^-$ react to form 1 mol of $Br_2$
First calculate the amount of moles of $"Br"_2$ formed when $65xx10^-3"g of bromide ions"$ react
Recall
$"grams of substance"/"molar mass of substance" = "moles"" "(1)$
$(65xx10^-3g) /("molar mass of Br"^-)$
Since the Br- has an extra electron
$"Molar mass of Br"^-) = "Molar mass of Br" + "Molar mass of electron"$
$(79.904g)/(mol) + (5.485 799 090 70 * 10^-10g)/"mol" = "79.9040000005g/mol"$
Now plug the variables equation (1)
$(65xx10^-3g) /(79.9040000005 ) = 0.00081347617"mol"$
ratio of bromine ions reacted to $"Br"_2$ formed is
$1mol : 0.5mol$
Therefore solving for ratio
$0.00081347617 : x = 1 : 0.5$
$0.00081347617 : 1/2*0.00081347617 = 1 : 0.5$
0.00040673808 mol of $"Br"_2$ is formed
We need 1kg of $"Br"_2$ so we have to find the no. of moles in 1kg of $"Br"_2$
Rearranging equation 1 we get
$"grams of substance" = "moles" xx "molar mass"$
$1000g = "molar mass of Br" xx 2 xx x$
$ 79.904*2x = 1000$
$159.808x = 1000$
$"moles" = 1000/159.808$
$= "6.25750901081moles"$
If $("0.00040673808 mol of Br"_2)/("1L of sea water")$
Then
$("6.25750901081moles of Br"_2)/("x") = ("0.00040673808 mol of Br"_2)/("1L of sea water")$
$x$ is the desired amount of sea water
$6.25750901081 = 0.00040673808x$
$x = 6.25750901081/0.00040673808$
$ 15384.6156003L$