3.1.2.5 balanced equations + associated calculations

if a salt’s formula is written like this - CuSO₄.5H₂O, what does it show about the salt?

that the salt is hydrated (the . shows water of crystallisation)

explain how you would determine the number of moles of water of crystallisation in a hydrated flask

• calculate the mass of the anhydrous salt

• calculate the mass of water

• determine the ratio of anhydrous salt : water by doing mass/Mr to calculate moles, then divide answers by smallest answer + find the simplest whole number ratio (empirical formula method)

how to determine the formula of a hydrated salt - RPA

describe the method you would carry out to find the formula of a hydrated salt. give the apparatus too

when hydrated salt is heated, water in it evaporates + leaves anhydrous salt. by comparing the mass before + after, the water evaporated can be calculated + x can be found

apparatus

• crucible + lid

• bunsen

• heat proof mat

• tripod

• clay-pipe triangle

• tongs

• access to a top pan balance

method

• weigh crucible + its lid on top pan balance, record mass in a table

• use a spatula to add required amount of hydrated salt to the crucible + reweigh (including lid), record the mass (all to 4 d.p)

• heat crucible over blue bunsen flame for approx 5 mins → remove lid of crucible at regular intervals to allow water to evaporate

• let cool on heatproof mat before reweighing, record mass

• heat crucible again for a further 5 mins, repeat step 4

• repeat steps 3 + 4 until a constant mass has been achieved

• calculate mass of hydrated salt + anhydrous salt → from this calculate mass of water evaporated

• calculate ratio of salt : water by finding the moles of each (mass/Mr) + divide my smallest answer

why must chemical equations be balanced before being used in calculations?

because the reacting ratios must be correct → it can then be used to calculate reacting masses, percentage yield + atom economy

what are the rules to balancing an equation?

• you cannot change or add any small numbers within a formula

• you can use large numbers in front of a formula

• large numbers can be x/2 or ‘halves’ if they are in front of a diatom eg ½ O₂

give the 4 state symbols

(s) solid

(aq) aqueous solution

(l) liquid

(g) gas

give the states of matter of:

a. metals

b. diatoms

c. ionic compounds

d. most simple molecules

e. acids

a. solids eg Na, Ca, Fe (exception: Hg)

b. mostly gases eg H₂, O₂ (exceptions: Br₂ I₂)

c. solids eg NaCl MgSO₄

d. gases eg CH₄ CO₂

e. aqueous eg HCl H₂SO₄

*when in solution, ionic compounds are aqueous

what will ionic equations only show?

the ions or species that change in a reaction → they leave out (omit) those ions that are not involved in the reaction

give the steps to writing an ionic equation

1. assign the correct state symbols

2. separate anything that is aqueous into the ions

3. cancel out anything that appears on either side of the equation

4. write the ionic equation

what is atom economy?

• a measure of what proportion of the products of a reaction are the desired product + how much is waste

the higher the atom economy the less the what?

the less the waste that is produced

high atom economy means advantages for who? + what type of advantages?

• economic, ethical + environmental advantages

• for society + industries developing chemical processes

give the equation to calculate atom economy (%)

(Mr of desired products/Mr of all products) x 100

what does the percentage yield tell us?

it tells us about the practical efficiency of the process

give 3 reasons why percentage yield is less than 100%

1. side reactions might occur : including reversible reactions, any unwanted ones

2. practical loss: eg during the transfer from one apparatus to another

3. incomplete reactions: when the reaction simply hasn’t finished

give the equation to calculate percentage yield

% yield = actual yield/theoretical yield x 100

rearrange the formula for % yield to find:

a. actual yield

b. theoretical yield

a. percentage yield x theoretical yield /100

b. actual yield x 100 /percentge yield

what is the theoretical yield? ie where does it come from?

it comes from the reacting masses/limiting reagents calculations → its the expected mass

10.0g of calcium carbonate was decomposed to give 3.60g of calcium oxide. what is the % yield of calcium oxide in this reaction? outline the steps + work out % yield

• work out theoretical mass

• substitute values into equation

→ answer = 64.3%

what is percentage purity?

its an application of percentage yield → it shows how much of the initial sample is pure

give the equation to calculate % purity

% purity = (actual mass/impure mass) x 100

what is the percentage purity of the Mg(NO₃)₂? (this can also be asked as ‘calculate the % by mass of magnesium nitrate in the sample’

2Mg(NO₃)₂ → 2MgO + 4NO₂ + O₂

• work out the moles of the product + use it + the mole to mole ratio work out the actual moles and then actual mass of Mg(NO₃)₂ that would have reacted

• substitute values into equation

• actual moles = 0.0434

• actual mass = 6.44g

→ answer = 80.5%

how is the concentration of a solution measured?

as the number of moles of solute (mol) per decimetre cubed of solution (dm³)

what is concentration also known as?

molarity

give the equation to calculate concentration + the units

concentration (moldm⁻³) = moles (mol) / volume (dm³)

c = n / V

how do you convert between cm³ to dm³ to m³

cm³ — /1000 → dm³ — /1000 → m³

cm³ ← x1000 — dm³ ← x1000 — m³

eg 7.55g of sodium chloride is dissolved in 500cm³ of water. what is the concentration of solution in moldm⁻³?

0.258moldm⁻³

how can you express concentration in g dm⁻³ from moldm⁻³?

conc in g dm⁻³ = conc in moldm⁻³ x Mr

because mass = moles x Mr

what is a standard solution? + what is its concentration usually given as?

a solution whose concentration is known accurately → conc is usually given in moldm⁻³ or g dm⁻³

when making up a standard solution what is it important to do? (give 2 points)

• to make sure that the mass of substance is accurately measured

• to make sure all the substance weighed out is successfully transferred to the volumetric flask used to make up the solution

describe the method to create a standard solution

1. transfer solid/sample to a weighing boat + record the mass

2. transfer solid into beaker + rinse boat with deionised water + transfer washings to beaker

3. add approx 100cm³ of deionised water to beaker + stir with glass rod until solid has dissolved

4. transfer solution to volumetric flask using funnel, rinse beaker, stirring rod + funnel with deionised water + transfer washings to volumetric flask

5. add deionised water to volumetric flask to make up to graduation mark (250cm³) → bottom of meniscus should be on mark

6. invert flask to make sure solution is thoroughly mixed

what is a titration?

a technique where a solution of known concentration is used to determine the concentration of an unknown solution

briefly outline what typically happens in a titration

• the standard solution (the known solution) is added from a burette to reaction volume of the unknown solution in the conical flask until the reaction is complete

• often, an indicator is used to signal the end of the reaction, the endpoint

give the colour in acid, in alkali + the endpoint for the indictors:

a. phenolphthalein

b. methyl orange

titrations of unknown solutions are done in two steps, what are these?

1. a rough titration used to determine the approx amount of standard solution needed to neutralise an unknown solution

2. subsequent more accurate titrations that you will use for your calculations

describe the method of a rough titration

1. rinse burette with deionised water, then with standard solution + then fill burette with standard solution, including the area below the tap (the jet), take the initial reading + record to 2 d.p ending in 5 or 0

2. rinse pipette with deionised water, then with unknown concentration, + then transfer 25cm³ of the solution from beaker into conical flask using pipette

3. add 3-4 drops of indicator to conical flask + place under burette → solution will change colour depending on its pH + indicator used

4. remove your filter funnel + add solution from burette, with constant swirling of conical flask, until the indicator changes colour (this indicates neutralisation) → you may add deionised water to sides of flask to ensure all solution has reacted

5. write down final burette reading to 2 d.p, ending in 5 or 0 → this is the volume of solution from burette required for neutralisation (titre value)

6. empty conical flask + wash thoroughly with deionised water

7. refill burette if necessary (if less than 25cm³ remains)

a. how can you ensure the area below the tap (the jet) is filled?

b. why do you need to do this?

a. by overfilling burette + then letting some solution out into waste beaker

b. if the jet is not filled, the titre volumes will be too high

why is it important to rinse pipette + burette with the solution it will be filled with?

it removes any drops of deionised water that would lower the concentrations of the solutions + affect the end point (titre value)

why can’t you add more than 3-4 drops of indicator?

• indicators are weak acids so anymore would affect the end point

a. why is a conical flask used?

b. why do you remove the filter funnel?

a. so no solution spills when swirling → none is lost

b. to ensure no additional drops enter from the burette + lower your titre

describe the method of the subsequent titrations

1. at start of titration, instead of adding a small mount of solution from your burette, add 2cm³ less than your previous titre value to your conical flask + then add drowse from your burette whilst swirling until you see the end point

2. close to your end point, you can use demonised water to wash the sides of your conical flask into solution to ensure all reagents are in solution reacting → this will not affect end point as water is not a reagent

3. continue with titration until you have concordant titre values (2 titres within 0.1cm³ of each other) → you only use the concordant results to calculate the average titre that is used in the calculations

describe an example of a titration between HCl (unknown conc) + NaOH (known conc)

what is the titre?

the volume of solution needed to neutralise the acid/alkali → can calculate by final reading - initial reading

how do you calculate the average titre to use in calculations?

• find the mean of the concordant results

outline the steps + calculate answer (concentration of NaOH)

after calculating the average titre (volume of HCl used) which is 24.90cm³, follow the steps in the image

as well as calculating the volume + concentration of an unknown in titrations using a standard solution, what else may you be asked to work out?

the original mass + relative molecular mass (Mr) in the standard solution

outline the steps + calculate answer

answer = 1.13g

every time you make a measurement with a piece of apparatus, there is what?

a small margin of error/uncertainty in that measurement due to the apparatus itself

• eg no balance can measure an exact mass, but a very precise balance may be able to measure a mass to the nearest 0.0001g whilst some may only measure to the nearest 0.1g

a. errors such as this (when using a piece of apparatus) are known as what? + cannot be _____

b. how can they be reduced?

a.

• apparatus error

• avoided

b. be using the most precise equipment available

what is the equation to work out the apparatus error for each piece of equipment?

what extra step would you have to do when calculating apparatus error when the apparatus us read more than once?

you would have to multiply the error by the number of readings eg when there’s a ‘change in temp’ or ‘change in mass’ as you have two readings

how can the amount of uncertainty in a practical be reduced without changing the equipment used?

by changing the quantity measured using that piece of equipment → by increasing the measured amount we can decrease % uncertainty

give 3 ways we could increase the titre volume (quantity measured) in a titration to lower the burette’s % uncertainty

1. increase volume of unknown solution (in conical flask)

2. increase concentration of unknown solution (in conical flask)

3. decrease concentration of known solution (in burette)

when is a back titration done?

to analyse a substance that is insoluble or only slightly soluble

how does a back titration work?

• a base (or acid) is treated with an excess of acid (or base) + the leftover acid (or base) is titrated

• you can then work back to find the original base (or acid) that reacted

BACK TITRATION EXAMPLE

Limeston his mainly calcium carbonate. A student wanted to find what % of some limestone is allowed to react with 100cm³ of 0.200 mol dm⁻³ HCl. The excess acid required 24.8cm³ of 0.100 mol dm⁻³ NaOH solution in a back titration.

Calculate the % of calcium carbonate in the limestone

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

HCl + NaOH → NaCl + H₂O

outline the steps + calculate the %

1. moles of NaOH = conc x volume

   0.1 x (24.8/1000) = 0.00248 mol

2. moles of leftover HCl = 0.00248 mol (1 : 1)

3. total moles of HCl = conc x volume

   0.2 x (100/1000) = 0.02 mol

4. moles of HCl reacted = total - leftover

   0.02 - 0.00248 = 0.01752 mol

5. moles of CaCO₃ → 1 : 2 ratio

   0.01752 / 2 = 0.00876 mol

6. mass of CaCO₃ = moles x Mr

   0.00876 × 100.1 = 0.877g

7. % purity = (actual m/impure m) x 100

   (0.877/1.00) x 100 = 87.7%