Use Of Amount Of Substance In Relation To Masses Of Pure Substances
Overview
A white crystalline solid arrives in a laboratory with nothing written on the jar. Nobody knows what it is. Within an hour an analyst can hand back its formula, and the only instrument that really mattered was a balance. That is a strange claim on the face of it: a balance reports masses, and a formula is a statement about how many atoms of each kind are locked together. Getting from one to the other is the single most useful piece of arithmetic in the whole of chemistry, and it is what this lesson is about.
You will start by learning to add up a formula, turning a string of symbols and subscripts into one number that stands for the mass of the whole unit. From there you will work out what fraction of a compound is the element you actually care about, which is how a farmer chooses between two fertiliser sacks and how a mining company decides whether an ore is worth digging. Then comes the reverse trip, and it is the best part: given nothing but masses measured on a bench, you will reconstruct the formula of a substance you have never met, then sharpen that formula until it names one compound and one compound only.
Objectives
- The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula. Students are expected to use relative atomic masses in the calculations specified in the subject content. Students should be able to calculate the relative formula mass (Mr) of a compound from its formula.
- The percentage by mass of an element in a compound can be calculated from the relative atomic mass of the element in the formula and the relative formula mass of the compound.
- The empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound. Students should be able to calculate empirical formulae and molecular formulae.
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Use Of Amount Of Substance In Relation To Masses Of Pure Substances. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- What is the relative formula mass of calcium carbonate, CaCO3? Relative atomic masses: C = 12, O = 16, Ca = 40. A. 68 B. 88 C. 100 D. 116 Answer: C
- What is the percentage by mass of oxygen in water, H2O? Relative atomic masses: H = 1, O = 16. A. 11.1 B. 16.0 C. 50.0 D. 88.9 Answer: D
- A compound contains 1.4 g of nitrogen and 0.3 g of hydrogen. What is its empirical formula? Relative atomic masses: H = 1, N = 14. A. NH B. NH2 C. NH3 D. N3H Answer: C
- Which of these is the empirical formula of glucose, C6H12O6? A. CHO B. CH2O C. C2H4O2 D. C3H6O3 Answer: B
- A compound has the empirical formula CH2 and a relative formula mass of 70. What is its molecular formula? Relative atomic masses: H = 1, C = 12. A. C2H4 B. C3H6 C. C4H8 D. C5H10 Answer: D
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