An alloy consists of rubidium and one of the other alkali metals. A sample of `4.6g` of the alloy when allowed to react with water, liberates `2.241dm^(3)` of hydrogen at `STP`.
Relative atomic masses: `A_(T)(Li)=7,A_(T)(Na)=23,A_(T)(K)=39,A_(T)(Rb)=85.5,A_(T)(Cs)=1.33`
What composition in `%` by mass has the alloy?

Answer

Concerning the molar masses of alkali metals, only lithium can come into consideration, i.e. the alloy consists of rubidum and lithium.
`n(Rb)+n(Li)=0.2mol`
`m(Rb)+m(Li)=4.6g`
n(Rb)M(rb)+n(Li)M(Li)=4.6g`
n(Rb)M(Rb)M(Rb)+(0.2-n(Rb))M(Li)=4.6`
`n(Rb).85.5+(0.2-n(Rb))7=4.6`
`n(Rb)=0.0408` mol
`n(Li)=0.1592` mol
`%Rb=(0.0408"mol"xx85.5gmol^(-1))/(4.6g)xx100=76`\