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Hertswood Academy Chemistry GCSE > C4- Chemical Calculations > Flashcards

C4- Chemical Calculations Flashcards

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1
Q
  1. What does the law of conservation of mass state?
A

It states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

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2
Q
  1. How can chemical reactions be represented?
A

They are represented by symbol equations which are balanced in terms of the numbers of atoms of each element involved on both sides of the equation.

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3
Q
  1. What is relative formula mass (Mr) of a compound?
A

This is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

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4
Q
  1. In a balanced chemical equation, what does the sum of the relative formula masses of the reactants equal?
A

The sum of the relative formula masses of the reactants equals the sum of the relative formula masses of the products in the quantities shown.

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5
Q
  1. Why do some reactions appear to show a change in mass?
A

One or more products may be a gas and the mass has not been taken into account as it has been ‘lost’ to the surroundings.

For example: when a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal, as the oxygen gas has not been taken into account. In thermal decompositions of metal carbonates, carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product.

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6
Q
  1. What are chemical amounts measured in?
A

Chemical amounts are measured in moles.

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7
Q
  1. What is the symbol for the unit of moles?
A

The symbol for the unit mole is mol.

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8
Q
  1. What is the mass of one mole of any substance equal to?
A

The mass of one mole of a substance in grams is numerically equal to its relative formula mass. For example, 1 mole of hydrogen has a mass of 1 gram.

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9
Q
  1. How many particles are in one mole of any substance?
A

The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.

One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance, for example, in one mole of carbon (C) the number of atoms is the same as the number of molecules in one mole of carbon dioxide (CO2).

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10
Q
  1. How can masses of reactants and products be calculated?
A

The masses of reactants and products can be calculated from balanced symbol equations.

Chemical equations can be interpreted in terms of moles. For example:
Mg + 2HCI MgCI2 + H2
Shows that one mole of magnesium reacts with two moles of hydrochloric acid to produce one mole of magnesium chloride and one mole of hydrogen gas.

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11
Q
  1. How are chemical equations balanced?
A

The balancing numbers in a symbol equation can be calculated from the masses of reactants and products by converting the masses in grams to amounts in moles and converting the numbers of moles to simple whole number ratios.

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12
Q
  1. Why is an excess of a chemical reactant used?
A

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used.

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13
Q
  1. What is the limiting reactant?
A

The reactant that is completely used up is called the limiting reactant because it limits the amount of products.

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14
Q
  1. How are quantities in solution measured?
A

The concentration of a solution can be measured in mass per given volume of solution, eg grams per dm3 (g/dm3).

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15
Q
  1. What two quantities are needed to calculate concentration?
A

The two quantities are mass of solute and volume of solution. Concentration is given by the equation:

Concentration =	Mass of solute
			Volume of solution
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16
Q
  1. Why is it sometimes not possible to obtain a calculated amount of product?
A

Even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of a product because:
• the reaction may not go to completion because it is reversible
• some of the product may be lost when it is separated from the reaction mixture
• some of the reactants may react in ways different to the expected reaction.

17
Q
  1. What is the amount of product obtained called?
A

The amount of a product obtained is known as the yield.

18
Q
  1. What is the yield when compared with the maximum theoretical yield called?
A

When compared with the maximum theoretical amount as a percentage, it is called the percentage yield.

%Yield = Mass of product actually made × 100
Maximum theoretical mass of product

19
Q
  1. What is atom economy?
A

The atom economy (atom utilisation) is a measure of the amount of starting materials that end up as useful products.

20
Q
  1. Why is atom economy important?
A

It is important for sustainable development and for economic reasons to use reactions with high atom economy.

21
Q
  1. Why a particular reaction pathway chosen to produce a specified product?
A

A specific pathway is used to ensure that yields and atom economy are maximised and the production of by-products is minimised.

22
Q
  1. What is the concentration of a solution measured in when moles are used in place of mass?
A

The concentration of a solution can be measured in moles per cubic decimetre (mol/dm3).

23
Q
  1. How can the number of moles of a solute or the mass of a solute be calculated from the concentration of a solution?
A

The amount in moles of solute or the mass in grams of solute in a given volume of solution can be calculated from its concentration in mol/dm3.

24
Q
  1. What is a titration?
A

The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator. If the volumes of two solutions that react completely are known and the concentration of one solution is known, the concentration of the other solution can be calculated.

25
Q
  1. What is the volume that a gas occupies at room temperature and pressure?
A

The volume of one mole of any gas at room temperature and pressure (20°C and 1 atmosphere pressure) is 24 dm3. Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.

26
Q
  1. How can the volumes of gaseous reactants be calculated?
A

The volumes of gaseous reactants and products can be calculated from the balanced equation for the reaction.

Hertswood Academy Chemistry GCSE (9 decks)

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