Topical Mcq Chemistry A level- Atoms, Molecules and Stoichiometry

Q1 .A sample of 0.025 mol of the chloride of an element Z was dissolved in distilled water and the solution made up to 500cm3. 12.5cm3 of this solution reacted with 25cm3 of 0.1 moldm-3 silver nitrate solution. What is the formula of the chloride?

A. Z2Cl    B. ZCL   C. ZCl2   D. ZCl4


Help:

Silver (I) Chloride reacts with Chloride to give silver (I) chloride percipitate : 

                                Ag+ (aq)    +   Cl- (aq) -------------> AgCl (s)

The amount of Aq+ used? = Concentration X volume

                                                       = o.1 (mol dm^-3)  x  (25/1000) (dm^3)
                                                      = 2.5 x 10^-3

Since 1 mole of Ag+ react with 1 mole of Cl- ,  Amount of Cl- present in 12.5 cm^3     =2.5x10^-3/12.5 x 500
                                                                                                                                                                             = 0.1 mol


Since 0.025 mole of compound contain o.1 mole of chloride, I mole of the compound therefore contain
0.1/0.025 = 4 mole of chloride. 

The Formula for the compound is ZCl4

Answer is D

ote

: 

Due to the presence of isotopes, the word “

average

” is necessary in the definition

 of 

A

r

. 

A

r

 has no unit, as it is a ratio of two masses. 

C

of 

atom

one

of

mass

the

 x 

12

1

element

an

of

isotope

an

of

atom

one

of

mass

mass

isotopic 

Relative