U1.4 · Composition of MixturesPractice · 混合物的组成
Lessons covered:U1.3 Elemental Composition of Pure SubstancesU1.4 Composition of Mixtures
- 1Numeric
A 2.00-g mixture of NaCl and KNO₃ is dissolved in water, and excess AgNO₃ solution precipitates 2.87 g of AgCl. What is the mass percent of NaCl in the mixture?
%ExplanationOnly NaCl supplies , so n(NaCl) = n(AgCl) = 2.87 ÷ 143.32 = 0.02003 mol, m(NaCl) = 0.02003 × 58.44 = 1.170 g, and the mass percent = 1.170 ÷ 2.00 × 100% = 58.5%.
Review: U1.4 Composition of Mixtures
- 2Multiple choice
Pure NaCl is 60.7% chlorine by mass. A sample labeled NaCl gives a lower percent of chlorine. Which impurity could explain this?
ExplanationA mixture’s percent Cl falls between those of its components, so only an impurity with a lower percent Cl than NaCl can pull the result down. Percent Cl: 83.6%, 47.6%, 74.5%, 63.9%. Only KCl is below 60.7%.
Review: U1.4 Composition of MixturesU1.3 Elemental Composition of Pure Substances
- 3Numeric
A 5.00-g mixture of NaHCO₃ and NaCl is heated to constant mass: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(g) + CO₂(g) (NaCl doesn’t react). The mass decreases by 1.24 g. What is the mass percent of NaHCO₃ in the mixture?
%ExplanationThe mass lost is the H₂O and CO₂ given off: every 2 mol of NaHCO₃ (168.02 g) loses 18.02 + 44.01 = 62.03 g. m(NaHCO₃) = 1.24 × 168.02 ÷ 62.03 = 3.359 g, so the mass percent = 3.359 ÷ 5.00 × 100% = 67.2%.
Review: U1.4 Composition of Mixtures
- 4Multiple choice
A gas mixture contains equal masses of CH₄ and C₂H₆. What is the mass percent of carbon in the mixture, approximately?
ExplanationC is 74.9% of CH₄ and 79.9% of C₂H₆. With equal masses of each, the mixture’s percent C is the average: (74.9% + 79.9%) ÷ 2 = 77.4%. A mixture’s composition always lies between its components’; 85.6% is the value for C₂H₄.
Review: U1.4 Composition of Mixtures
- 5Multiple choice
Brass is an alloy of copper and zinc. Which observation best shows that brass is a mixture rather than a compound?
ExplanationA compound has a fixed composition (law of definite proportions); a mixture’s composition can vary. Brass with different copper contents shows it’s a mixture. Conductivity and color are properties that don’t tell you whether the composition is fixed, and compounds can contain two elements too.
Review: U1.4 Composition of Mixtures
- 6Multiple choice
The graph shows the mass percent of Cl in mixtures of and against the mass percent of . A sample is 55.4% Cl by mass. About what percent of the sample is ?
ExplanationThe line runs from pure KCl (47.55% Cl) to pure NaCl (60.66% Cl). Reading across from 55.4% on the vertical axis meets the line at about 60% on the horizontal axis. Check: (55.4 − 47.55) ÷ (60.66 − 47.55) = 0.60. Choosing 40% gives the percent of KCl instead.
Review: U1.4 Composition of MixturesU1.3 Elemental Composition of Pure Substances
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