計數題*

小霞的fans

the formula for hydrated iron(II) sulphate is FeSO4．xH2O. On strong heating , 20.1 g of the sulphate

produces 9.1g of water. what is the value of x ?

(relative atomic masses: H = 1.0 , O = 16.0 , S=32.1 , Fe=56.0

A.5
B.6
C.7
D.8

我完全唔識計 = = " D書既example如果計X既話會比埋%架 ..
呢題冇比.. 唔識計= =

鴨

Equation
FeSO4．xH2O(s) -> FeSO4(s) + xH20(g)

No. of moles of H2O gas = 9.1/18 = 0.506 moles

No of moles of FeSO4．xH2O = 0.506/x moles

mass/molar mass = no of moles
20.1/(56+32.1+16X4+(18x)) = 0.506/x
                                                x = 7.003  ~7

so the formula is FeSO4．7H2O

inthomaspy

% by mass of H2O in the sample = 9.1/20.1 = 0.453
hence
[ 1(2) + 16 ] x / { 56.0 + 32.1 + 16 (4) + [1(2) + 16] x } = 0.453
18 x / ( 152 + 18x ) = 0.453
18 x = 68.856 + 8.154 x
x = 6.99 = 7
Ans = C

%可以自己計出來