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Explain how a mixture of two pure substances differs from a single pure substance at the particulate level?
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A pure substance contains only one type of atom, molecule, or formula unit, so every particle in the sample is identical. A mixture contains atoms, molecules, or formula units of two or more different types, and the relative proportions of these types can vary from one sample to another.
Predict how the percent composition by mass of carbon in a mixture of CH₄ and CO₂ changes if additional CO₂ is added? Justify your prediction.
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The percent composition by mass of carbon in the mixture decreases. CH₄ is 74.87% carbon by mass, while CO₂ is only 27.29% carbon by mass. Adding more CO₂ increases the fraction of the mixture that has a lower carbon mass percent, so the overall carbon mass percent of the mixture decreases.
Describe how elemental analysis can be used to determine the purity of a sample of a known compound?
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The theoretical percent composition by mass of each element in the pure compound is calculated from its chemical formula and molar mass. The experimental mass percentages are then measured through elemental analysis. If the experimental values match the theoretical values, the sample is pure. If they differ, the sample contains an impurity — it is a mixture rather than a pure substance.
Explain why two different pure compounds cannot have identical percent compositions by mass if they have different empirical formulas?
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The percent composition by mass is determined solely by the types of atoms and their ratio in the empirical formula. Different empirical formulas mean different atomic ratios, which produce different mass contributions from each element relative to the total formula mass. Therefore, two compounds with different empirical formulas must have different percent compositions by mass.
A 8.00 g mixture of MgO and CaO is analyzed and found to contain 2.85 g of oxygen. Calculate the mass of MgO in the mixture.
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Mass fraction of O in MgO: $$\frac{16.00}{40.30}=0.3970$$ Mass fraction of O in CaO: $$\frac{16.00}{56.08}=0.2853$$ Let x = mass of MgO: $$0.3970x+0.2853(8.00-x)=2.85$$ Expand: $$0.3970x+2.282-0.2853x=2.85$$ Combine like terms: $$0.1117x=0.568$$ Solve: $$x=\frac{0.568}{0.1117}$$ $$x=5.09\text{ g MgO}$$ The mixture therefore contains 5.09 g MgO and 2.91 g CaO.
Predict whether the mass percent of magnesium in a mixture of MgO and MgCl₂ increases or decreases as the proportion of MgCl₂ in the mixture increases? Justify your prediction.
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The mass percent of magnesium decreases. MgO contains 60.32% Mg by mass (24.31/40.30), while MgCl₂ contains only 25.53% Mg by mass (24.31/95.21). Increasing the proportion of MgCl₂ adds more of the component with the lower magnesium mass percent, so the overall mass percent of magnesium in the mixture decreases.