quantitative chemistry

When should you use methyl orange?

When titrating a strong acid with a weak base.

When should you use phenolphthalein?

When titrating a weak acid with a strong base.

What is an advantage of dot and cross diagrams.

Because we use dots to represent electrons from one atom and crosses to represent electrons from the other atom, it's very easy to tell where the electrons are coming from.

What are the disadvantages of dot and cross diagrams?

Dot and cross diagrams don't tell us about the bonds between the ions or the size of the atoms

What is the charge and formula for Hydroxide ions?

OH ^-

What is the charge and formula for Sulfate ions?

SO₄^2-

How can the accuracy of a titration be increased?

• By swirling the solution.
• Add acid/alkali drop wise near the end point.
• Repeat it and calculate the mean.

Why do some reactions appear to have a gain in mass?

• If one of the reactants is a gas that is found in the air and you are using an unsealed vessel then you can't account for its mass.

• When the gas reacts to form part of the product, it becomes contained inside the reaction vessel - so the total mass of the chemicals inside the reaction vessel increases.

Why may the mass of reactants appear to decrease after a chemical reaction?

• One of the products may be a gas and all the reacts anre solid, liquid or aqueous.

• Before the reaction, all the reactants are contained in the reaction vessel.

• If the vessel isn't enclosed, then the gas can escape from the reaction vessel as it's formed.

• It's no longer contained in the reaction vessel, so you can't account for its mass.

What is a mole?

unit of measurement used to express the amount of a substance in a sample equal to avogadro's constant.

what’s avogadros constant?

6.02 * 10²³

What is the fomula for the number of moles?

Number of moles = mass in g (of element or compound) / Mr (of the element or compound)

What is the formula for the percentage mass of an element in a compound?

Percentage mass of an element in a compound} = Ar number of atoms of that element / Mr of the compound}} 100

How do you balance an equation using reacting masses?

• Divide the mass of each substance by its relative formula mass to find the number of moles.

• Divide the number of moles of each substance by the smallest number of moles in the reaction.

• If the numbers aren't whole numbers, multiply all the numbers by the same amount so the all become whole numbers.

How do you find the volume of any gas at room temperature?

volume of gas = number of moles of gas * 24

How much volume does 1 mole of any gas occupy at room temperature and room pressure?

24dm³

How much is 1dm³ in cm³?

1000cm³

What is the formula for atom economy?

Atom economy = (mass of desired product / total mass of products) x 100

What are the advantages of having a high atom economy?

• It is better for the environment

• It is better for profits

Why can having a low atom economy be bad for the environment?

• A lot of waste gets produced that has to be disposed somehow.

• Resources get used up quickly.

Why can having a low atom economy be bad for profits?

• Raw materials are expensive to buy.

• Waste products can be expensive to remove and dispose of responsibly.

What reactions have an atom economy of 100%?

Reactions that only have one product.

How can you reduce the amount of waste products in a reaction?

By using reactions which give useful "by-products".

What other factors should be considered when choosing a reaction to use?

• Percentage yield.

• Rate of reaction.

• Position of equilibrium for reversible reactions.

• By-products made.

What is percentage yield?

The percentage yield shows how much useful product is obtained compared to the maximum possible mass.

What is the formula for percentage yield?

PercentageYield=ActualYield(g)TheoreticalYield(g)×100

 

Why are yields always less than 100%?

• Not all reactants react to make a product. For example in reversible reactions, the products can turn back into reactants, so the yield will never be 100%.

• There might be some side reactions for example with gases in the air or impurities in the mixture.

• You lose some product when you seperate it from the reaction mixture for example when transfering it from one container to another or when you filter it.

What are the first two steps for carying out a titration to find the concentration of an alkalai?

• Using a pipette add a set volume of alkalai into a concical flask.

• Add three drops of indicator.

What are the second two steps for carrying out a titration to find the concentration of an alkali?

• Use a funnel to fill a burette with some acid of known concentration.

• Record the inital volume of the acid in the burette.

What are the final steps for carrying out a titration to find the concentration of an alkali?

• Using the burette, add the acid to the alkali a bit at a time - giving the conincal flask a regular swirl. Go especially slow when you think the end-point is about to be reached.

• Record the final volume of acid in the burette and use it, along with the inital reading, to calculate the volume of acid used to neutralise the alkalai.

How many times should you repeat a titration?

You should repeat the titration until you get three concordant answers (within 0.1cm³).

Why is universal indicator not used during titrations?

Because a sudden change is needed to see the end point.

What indicators are commonly used in titrations?

• Litmus.

• Methyl orange.

• Phenolphthalein.

What colours does litmus indicator change in acidic and alkali conditions?

red in acidic conditions.

Purple in alkali conditions.

What colours does phenolphthalein indicator change in acidic and alkali conditions?

Colourless in acidic conditions.

Pink in alkali conditions.

What is a limiting reactant?

The reactant in a reaction that's completely used up and so limits the amount of product formed.