Ch.3 - Mass Relationships in Chemical Reactions

Problem 88b

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Chapter 3, Problem 88b

What are the empirical formulas of substances with the following mass percent compositions? (b) Ilmenite (a titanium-containing ore): 31.63% O, 31.56% Ti, 36.81% Fe

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Convert the mass percentages to grams, assuming you have 100 grams of the substance. This means you have 31.63 grams of O, 31.56 grams of Ti, and 36.81 grams of Fe.

Convert the grams of each element to moles by dividing by their respective molar masses: O (16.00 g/mol), Ti (47.87 g/mol), and Fe (55.85 g/mol).

Determine the mole ratio of the elements by dividing each element's mole value by the smallest number of moles calculated in the previous step.

If necessary, multiply the mole ratios by a whole number to get whole numbers for each element in the formula.

Write the empirical formula using the whole number mole ratios as subscripts for each element.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Empirical Formula

An empirical formula represents the simplest whole-number ratio of the elements in a compound. It is derived from the mass percent composition of the elements, allowing chemists to determine the relative amounts of each element present in the substance. For example, if a compound contains 50% carbon and 50% oxygen, its empirical formula would be CO.

Mass Percent Composition

Mass percent composition is the percentage by mass of each element in a compound. It is calculated by dividing the mass of each element by the total mass of the compound and multiplying by 100. This information is crucial for determining the empirical formula, as it provides the necessary data to find the ratio of elements present in the substance.

Molar Mass

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is essential for converting mass percent compositions into moles of each element, which is necessary for determining the empirical formula. For instance, knowing the molar masses of titanium (Ti), iron (Fe), and oxygen (O) allows for the calculation of the number of moles of each element from their respective mass percentages.

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Molar Mass Concept

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