Chemistry Labs

Problem 4

An alloy consists of rubidium and one of the other alkali metals. A sample of 4.6 g of the alloy, when allowed to react with water, liberates 2.241 dm3^3 of hydrogen at STP. (a) Which alkali metal is the component of the alloy? (b) What is the composition of the alloy in % by mass? Relative atomic masses: Ar(Li)=7A_r(\text{Li}) = 7; Ar(Na)=23A_r(\text{Na}) = 23; Ar(K)=39A_r(\text{K}) = 39; Ar(Rb)=85.5A_r(\text{Rb}) = 85.5; Ar(Cs)=133A_r(\text{Cs}) = 133.
Step 1 of 4: Total moles of metal
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n(HX2)=2.24122.41=0.1 mol⇒n(M)=0.2 moln(\ce{H2}) = \dfrac{2.241}{22.41} = 0.1\ \text{mol} \Rightarrow n(\text{M}) = 0.2\ \text{mol}
Analysis

All alkali metals react as 2 M+2 HX2O→2 MOH+HX2\ce{2M + 2H2O -> 2MOH + H2}, so each mole of M releases half a mole of H2; 0.1 mol H2 means 0.2 mol of metal in total.