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SQA Higher Chemistry Mass calculations

Mass calculations

This page covers the following topics:

1. Percentage mass
2. Theoretical mass
3. Atom economy

Percentage mass is a way of representing the percentage of an element in a compound. Mass percentage is calculated by the mass of the element divided by the total mass of the compound.

Percentage mass

Due to incomplete reactions, when experiments are carried out there is not a 100% yield of products. The theoretical yield is the amount of product you could produce theoretically. The percentage yield is a measure of the amountthe actual yield you get from an experiment.

Theoretical mass

Atoms are not gained or lost in a chemical reaction, some atoms from the reactants end up in the product, and some in by-products. The atom economy of a reaction expresses how efficiently a reaction makes use of reactant atoms, i.e. how much product is creating in comparasion to by-product. Atom economy is a percentage, thus the highest it can possibly be is 100%.

Atom economy

1

The production of ethanol via glucose fermentation has relatively low atom economy. How could the atom economy of this reaction be improved?

The production of ethanol via glucose fermentation has relatively low atom economy. How could the atom economy of this reaction be improved?

2

Element Z has a percentage mass of 65.3% in the compound Hβ‚‚SZβ‚„ (molecular mass = 98). What is element Z?

Element Z has a percentage mass of 65.3% in the compound Hβ‚‚SZβ‚„ (molecular mass = 98). What is element Z?

3

3 g of C₃H₇OH has reacted with an excess of CH₃COOH (Ar = 60 g/mol) to give 1.8 g of C₃H₇OOCCH₃ (Ar = 102 g/mol) and some water, what is the actual yield of this reaction?

3 g of C₃H₇OH has reacted with an excess of CH₃COOH (Ar = 60 g/mol) to give 1.8 g of C₃H₇OOCCH₃ (Ar = 102 g/mol) and some water, what is the actual yield of this reaction?

4

What is the atom economy for the production of Nickel in the reaction NiO + C β†’ Ni + CO ?

What is the atom economy for the production of Nickel in the reaction NiO + C β†’ Ni + CO ?

5

30.0 g of KClO₃ is heated until it completely decomposes into KCl + Oβ‚‚ via the following reaction: 2KClO₃ β†’ 2KCl + 3Oβ‚‚. What is the theoretical yield of oxygen gas in this experiment?

30.0 g of KClO₃ is heated until it completely decomposes into KCl + Oβ‚‚ via the following reaction: 2KClO₃ β†’ 2KCl + 3Oβ‚‚. What is the theoretical yield of oxygen gas in this experiment?

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