amount of substance

Question 1

what is the relative atomic mass?

Answer 1

the average mass of an atom or isotope compared to the mass of carbon 12

Question 2

what is the relative formula/ molecular mass?

Answer 2

the mass of a molecule of compared to 1/12 the relative atomic mass of one atom of carbon 12

Question 3

what is Avogadros constant?

Answer 3

1 mole = 6.022x10 23 atoms /particles/molecules of something.

Question 4

how do you calculate the moles of a substance?

Answer 4

moles = mass/mr

Question 5

how do you figure out the moles of an equation and the number of moles they will make?

Answer 5

1. use the molar ratio given in the question
2. divide the number of moles you’re given by the number of moles in the question to find the scale factor
3. times the other moles in the equation b the scale factor to find how many moles would be made

Question 6

how do you figure out the mass of a reactant or product that will be made?

Answer 6

1. find out the moles of the element of the mass that you know using. mass = Mr x moles
2. use the molar ratio given in the balanced equation to figure out the moles of the element you’re looking for.
3. once the moles of the element your looking for have been found use this to find out the mass by using moles = mass/mr rearranged/

Question 7

how do you figure out the limiting reagent in a question?

Answer 7

1. look at the amount of moles needed for the reactants to react fully
2. look at the moles of the reactants and decide which is in excess, and which one is limiting
3. the limiting reagent will have to figure out how many moles will react and how many will be left over you can use this to figure out how many moles will react and how many will be left over.
4. if you’re given the masses you can figure out the moles by using moles = mass/ mr

Question 8

how do you measure the mass of the solid used for the volumetric solution?

Answer 8

1. Zero the balance, then place a weighing boat onto the balance and record its mass
2. add the rough amount of solid to the boat and record the mass
3. add the solid mass to a beaker and record the mass of the boat again
4. if you subtract the mass of the weighing boat and solid by the mass of the weighing boat by itself, you should get the mass of the solid. this is called weighing by difference

Question 9

how do you make a volumetric solution?

Answer 9

• into a small beaker, add 50 cm^ of distilled water and add the solid, stirring until completely dissolved.
• then transfer the contents of the beaker into a 250 cm^3 volumetric flask, don’t filter it. rinse the beaker and add the washing
• then slowly add more water using a dropper until the solution reaches the calibration mark at 250 cm^3
• add the stopper and invert 10 times to mix the solution thoroughly.
• to find out the concentration do conc. = moles/volume

Question 10

how do you calculate the uncertainty?

Answer 10

uncertainty = error value of equipment/value measured x 100
- you must remember how many times you used the balance, then times the error value by this number

Question 11

how do you calculate the concentration of a solution?

Answer 11

concentration = moles /volume.

Question 12

how do you convert the volume from cm^3 into m^3?

Answer 12

times by 10 -6

Question 13

how do you calculate the experimental error of an experiment?

Answer 13

experimental error = measured value - real value / real value
- if this is greater than the total apparatus uncertainty the the experiment is inaccurate.

Question 14

how do you perform a tiration?

Answer 14

• use the pipette and pipette filler to add a 25 cm^3 volume of sodium hydroxide solution to a clean conical flask.
• add a few drops of indicator and put the conical flask on a white tile to see the colour change better
• fill the burette with HCl and note the starting volume
• slowly add the acid from the burette to the alkali in the conical flask, swirling constantly to mix
• stop adding the acid when the endpoint is reached, (when the indicator first changes colour) and not the final volume on the burette
• calculate the titre by subtracting the starting value on the burette from the end value.
• repeat the steps above until you get concordant results (results within 0.01 of each other) and do drop by drop near the end.

Question 15

How do you work out the concentration of an acid/alkali using the results of a titration?

Answer 15

1. write out the equation and balance it.
2. calculate the number of moles of the element that you know the concentration and volume of. don’t forget to convert the concentration form cm^3 into dm ^3 by dividing by 1000
3. looking at the molar ratio in the equation find out the moles of the acid/ alkali you are looking for., then to find out the concentration, use the equation concentration = moles /volume.
   9 to find the volume rearrange the equation.

Question 16

What is the ideal gas equation?

Answer 16

p(Pa)V (m^3)= nRT pressure x volume (m^3) = number of moles x gas constant x temperature (k) (dont forgert to convert into standard conditions)

Question 17

when working out the volume of a solution what must you convert it to?

Answer 17

cm^3 -> Dm^3 by dividing by 1000

Question 18

what is the empirical formula?

Answer 18

the simplest whole number ratio of elements in a compund.

Question 19

how do you calculate the empirical formula?

Answer 19

1. write out the elements involved
2. write the percentages or masses down
   3, divide the percentages/ masses by the me to get the number of moles
3. then divide by the smallest one to get the ratios
4. you can round if the number is close to another

Question 20

how do you calculate the molecular formula ?

Answer 20

1. work out the mr of the empirical formula
2. divide this number by the mr of the molecular formula
3. use this number and multiply by all the atoms in the empirical formula to get molecular formula.

Question 21

What is the theoretical yeild?

Answer 21

the amount of a product produced assuming no products are lost an all reactants react fully.

Question 22

how do you calculate the theororetical yield?

Question 23

how do you calculate the percentage yield?

Answer 23

percentage yield = actual yield /theorectical yield x 100

Question 24

why is perecentage yield never 100%?

Answer 24

• not all of the reactants may have reacted
• ## if product is a gas it may be lost to the environment

Question 25

what is the atom economy?

Answer 25

How efficient a reaction is.

Question 26

how do you calculate atom economy?

Answer 26

% atom economy = molecular mass of product/sum of molecular masses of all reactants x 100
(remember you may need to write the equation out first and balance it also when finding the mss of desired product you must times by the moles infront of it)

Question 27

Why is atom economy important?

Answer 27

• companies will try to use reactions that tend towards 100% atom economy
• high atom economies produce less waste and so benefit from the envrionment
• high atom economies means that raw materials are used more efficiently. this is more sustainable
  -higher atom economy means less by- products so less time spent seperating these from desirable products.