# Calculations Involving Masses

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What is the law of the conservation of mass?
During a chemical reaction, no atoms are destroyed and no atoms are created.
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How can you tell that mass has been conserved in a closed system?
The total mass of the reactants is the same as the total mass of the products
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Why might you observe an increase in mass in a non-enclosed system?
At least one of the reactants is a gas that's found in air. Before the reaction, the gas is in the air, but isn't contained in the vessel, therefore it isn't measured and included in the mass. It becomes contained in the vessel when it reacts.
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Why might you observe a decrease in mass in a non-enclosed system?
At least one of the products is a gas and they are contained in the vessel before the reaction. Then when they react, as the vessel isn't enclosed, they can escape as it is formed. You can't measure its mass then as it isn't in the vessel anymore.
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How do you calculate the relative formula mass?
Add the relative atomic masses of all the atoms in its formula together.
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What is the molecular formula?
The actual number and type of each atom in a compound.
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What is the empirical formula?
The smallest whole number ratio of atoms in a compound.
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How do you calculate the empirical formula?
Divide the numbers in the molecular formula by the largest number that goes into all the numbers exactly.
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How do you convert the empirical formula into molecular formula using RFM?
Start by finding the relative formula mass of the empirical formula. Divide the Mr of the compound by the Mr of the empirical formula. Multiply everything in the empirical formula by the result.
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6.02 x 10^23
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How do you calculate the number of moles?
Number of moles = mass in g/Mr of compound or Ar of element
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How do you calculate empirical formula from a mass?
Work out how many moles of each element you have. Then calculate the smallest whole number ratio between the number of moles of each atom to get the empirical formula.
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How do you calculate the mass of a product made in a chemical reaction from a given mass of reactant?
Balanced equation. Work out Mr of reactant and product concerned. Multiply by balancing numbers. Work out how much product would be made from 1g reactant- divide by Mr of reactant. Multiply no of grams of product formed by acc number of g used.
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How do you work out the limiting reagent?
Calculate no of moles of each substance. Divide by smallest no of moles. Compare ratios between moles of the products with the balanced symbol equation.
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What is the limiting reagent?
The reactant that is used up in the reaction
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What is the formula for concentration?
Concentration = mass of solute/volume of solution
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How many cm^3 in a dm ^3?
1000
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## Other cards in this set

### Card 2

#### Front

How can you tell that mass has been conserved in a closed system?

#### Back

The total mass of the reactants is the same as the total mass of the products

### Card 3

#### Front

Why might you observe an increase in mass in a non-enclosed system?

### Card 4

#### Front

Why might you observe a decrease in mass in a non-enclosed system?

### Card 5

#### Front

How do you calculate the relative formula mass?