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 4 of 4: Mass percentages
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w(Rb)=0.0408×85.54.6≈76 %;w(Li)≈24 %w(\text{Rb}) = \dfrac{0.0408 \times 85.5}{4.6} \approx 76\ \%; \quad w(\text{Li}) \approx 24\ \%
Analysis

Masses: m(Rb)=0.0408×85.5≈3.49m(\text{Rb}) = 0.0408 \times 85.5 \approx 3.49 g (≈76\approx 76 %) and m(Li)=0.1592×7≈1.11m(\text{Li}) = 0.1592 \times 7 \approx 1.11 g (≈24\approx 24 %).