formulae + equations

Question 1

define empirical formula

Answer 1

the simplest whole number ratio of the elements present in one molecule or formula unit of the compound

Question 2

how to calculate the emperical formula

Answer 2

• lay out in table with mass, mr and moles
• caluclate the moles
• divide by smallest number of moles to obtain ratio
• when in a percentage assume it as mass

Question 3

define molecular formula (mr)

Answer 3

the exact numbers of atoms of each element present in the formula of the compound

Question 4

how to calclate the mr

Answer 4

mass = mr x moles
mr = mass/moles
moles = mass/mr

Question 5

what is the ideal gas equation

Answer 5

PV = nRT
n = pv/rt
P = pressure in pascals (Pa)
V = volume in cubic metres (m3)
n = the amount of substance in moles (mol)
R = the gas constant, 8.31 J mol-1 K-1
T = temperature in Kelvin (K) (273+C)

Question 6

percentage of an element in a compound

Answer 6

(ar x no. of atoms in the element/mr of compound) x 100

Question 7

displacemnt reactions

Answer 7

more reactive element displaces the less reactive element

Question 8

neutralisation reaction

Answer 8

can be identified by the presence of reactant acids and bases as well as the formation of a neutral salt solution and water

Question 9

acid reactions

Answer 9

acid + metal → salt + hydrogen.
acid + base → salt + water.
acid + carbonate → salt + water + carbon dioxide.
acid + hydrogencarbonate → salt + water + carbon dioxide.
acid + ammonia → ammonium salt.

Question 10

define a mole

Answer 10

• the mass of substance that contains the same number of fundamental units as exactly 12.00g of carbon-12
• one mole of any element is equal to the relative atomic mass of that element in grams eg. one mole of carbon (6.02 x 10^23) is 12.00 g

Question 11

define avogadros constant

Answer 11

• the number of particles equivalent to the relative atomic mass or molecular mass of a substance
• 6.02 x 10^23 g mol-1

Question 12

types of errors

Answer 12

• random errors
• systematic errors

Question 13

systematic errors

Answer 13

• errors that occur as a result of a faulty or poorly designed experimental procedure
• will always pull the result away from the accepted value in the same direction (always too high or always too low)
• ## Repeating the experiment and working with the average value will not remove any systematic errors

Question 14

random errors

Answer 14

• will pull a result away from an accepted value in either direction (either too high or too low)
• Repeating the experiment and working with the mean average of the results can help to reduce the effects of random errors

Question 15

calculatin errors

Answer 15

balance +- 0.005g x 2 (weighing by difference)
volumetric flask +- 0.1cm3
pipette +-0.06cm3
burette +- 0.05cm3x 2 (weighing by dufference)

Question 16

% appartus error =

Answer 16

margin of error/quantity measured x 100

Question 17

% experimental eroor =

Answer 17

real ans- exoerimental ans/ real ans x 100

Question 18

percentage yield

Answer 18

(actual yield/theoretical yield) x 100

Question 19

atom economy

Answer 19

(mr of desired/sum mr of all reactants) x 100

Question 20

percentage purity

Answer 20

mass of pure chemical/total mass of smaple x100

Question 21

no of particles =

Answer 21

no of moles x 6.022x10^23

Question 22

soluble salyts

Answer 22

• all sodium, potassium and ammonium salts
• all nitrate salts
• sulphate salts- except calcium barium and leas sulphate
• chloride salts - except silver and lead chloride
• sodium, potassium & ammonium carbonates & hydroxides

Question 23

insoluble salts

Answer 23

• calcium sulphate, barium sulphate and lead sulphate
• silver chloride and lead chloride
• silver bromide, silver iodide (silver halides)
• all other carbonates and hydroxides

Question 24

why is perecentage yield is not always 100%

Answer 24

• some reactant/product may be lost in transfer between equipment
• side-reactions convert sme reactants or unwanted by-products
• reaction migth not go to completen
• reaction is reversible
• reactanst could be impure

Question 25

titration calcs

Answer 25

1. Calculate the concentration of your sulfamic acid solution. The Mr of sulfamic acid is 97.1. 2. Calculate the mean titre using your concordant results. 3. Calculate the number of moles of sulfamic acid in your pipette. 4. Calculate the concentration of the sodium hydroxide solution used. 5. Sulfamic acid is a monoprotic acid. One mole of sulfamic acid will react exactly with one mole of sodium hydroxide.

Question 26

titration step by step

Answer 26

**reparing the burette:** Rinse a burette with distilled water followed by the solution being used in the burette (the solution we are determining the concentration of). Fill the burette with the solution being used and run solution through the tap to ensure that the tap is full. Check for air bubbles in the tap, and remove any found. **making a standard solution:** Accurately weigh out the appropriate mass of solute by · weigh the mass of an empty weighing boat, · adding the appropriate mass of solute to the boat and re-weigh, · empty the solute into a 250cm3 beaker, · re-weigh the empty weighing boat. Dissolve all of the solute in the beaker by adding 75-100cm3 of solvent (commonly distilled water) and stir using a glass rod. Break up the solid using the end of the glass rod. Rinse out the volumetric flask with distilled water and then carefully pour the solution into the volumetric flask (commonly a 250cm3 vol. flask) using a funnel. Carefully rinse the glass rod into the vol. flask and rinse the inside of the beaker numerous times and pour these washings into the vol. flask. Be careful not to add too much solvent during rinsing. Carefully make the solution up to the mark using a clean dropping pipette to ensure that the base of the meniscus is on the mark. Place a lid on the volumetric flask and invert the flask at least ten times. **Pipetting out a known aliquot of the standard solution:** Rinse a glass pipette with distilled water followed by a small amount of the standard solution. Do not put a wet pipette into the volumetric flask, but pour some of the standard solution into a beaker and rinse using that solution. Accurately pipette out 25.0cm3 of the standard solution using a pipette filler and your thumb to lower the meniscus until the base of the meniscus is on the mark. Hold the pipette over the conical flask and allow the solution to drain into the flask. Touch the end of the pipette to the glass base of the conical flask 4 times. **Carrying out the titration:** Add 3-4 drops of indicator to the standard solution in the conical flask and swirl. Place a white tile underneath the conical flask to aid with identifying the colour change at the end point. Carry out a rough titration first by: · recording the initial volume (to 2 decimal places) on the burette, · add solution from the burette to the conical flask with continual swirling of its contents until the solution permanently changes colour. · record the final volume (to 2 decimal places) and determine the volume added - this volume will be approximately 1-2cm3 past the end point of the titration. Carry out further, accurate titrations by repeating the method about but: · 1-2cm3 from the end point, add the solution from the burette one drop at a time with swirling. Repeat this until you have three concordant results (ones that are within 0.2cm3 of one another).

Question 27

heating to dryness

Answer 27

Weigh a crucible, record this mass value in your results table. 2. Add between 1.30 g and 1.50 g of hydrated crystals. Reweigh the crucible, and record the new mass in the results table. 3. Place the crucible containing the hydrated crystals on the pipe clay triangle and gently heat for two minutes. 4. Allow to cool and weigh the crucible and its contents, record the mass. 5. Reheat the crucible and its contents and reweigh, record the new mass. 6. Continue reheating and reweighing until constant mass is observed. This process is known as ‘heating to constant mass’ and ensures that all the water of crystallisation has been removed from the hydrated crystals.