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 2 of 4: Identify the second metal
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Mˉ=4.6 g0.2 mol=23 g mol−1⇒the second metal is Li\bar{M} = \dfrac{4.6\ \text{g}}{0.2\ \text{mol}} = 23\ \text{g mol}^{-1} \Rightarrow \text{the second metal is Li}
Analysis

The mean molar mass 23 g mol−1^{-1} must lie between those of the two metals; since Ar(Rb)=85.5A_r(\text{Rb}) = 85.5, the partner must be lighter than 23 — only Li (7) qualifies.