topic 2: amount of substance

Question 1

ideal gas equation

Answer 1

pV= n RT

Question 2

temperature into kelvins

Answer 2

+273

Question 3

m3 —> dm3 —> cm3

Answer 3

m3—-> dm3 (x1000)
dm3—-> cm3 (x1000)
m3—–> cm3 (x1000000)
cm3—–>dm3 (divide by 1000)
dm3—–> m3 (divide by 1000)
cm3—->m3 (divide by 1000000)

Question 4

percentage atom economy

Answer 4

(molecular mass of desired product/ sum of molecular masses of all reactants) x 100

Question 5

what is atom economy?
why is a higher atom economy desirable?

Answer 5

-atom economy is a measure of the efficiency of a reaction

in industrial processes, it is desirable to have a high atom economy for a reaction:

-this means there is little or no waste products, only the desired product

Question 6

atom economy benefits for the environment and economy

Answer 6

-processes with high atom economy is more economically viable in the chemical industry
-environmentally friendly, less waste produced and less energy used
-more efficient use of raw materials, so they are more sustainable

Question 7

moles formula

Answer 7

mass= moles x Mr
moles= volume x concentration

Question 8

percentage yield

Answer 8

(actual mass produced/ theoretical mass) x 100

Question 9

what is percentage yield?

Answer 9

-the percentage yield tells us how efficient the process was of which reactants are converted into products and how much was lost by practical process of obtaining a product

-theoretical yield: mass of product that should be formed in a chemical reaction
-actual mass: less than the theoretical yield

Question 10

causes of a lower percentage yield

Answer 10

-due to incomplete reactions
-side reactions
-loss in of product in transfer

Question 11

RP1 - Outline how to prepare a standard solution in a volumetric flask (for acid-base titration).

Answer 11

1. Weigh the sample bottle containing the solid on a (2 dp) balance in a weighing boat.
2. Transfer solid to beaker
3. reweigh the weighing boat and record the difference in mass.
4. Add distilled water and stir with a glass rod until all the solid has dissolved.
5. Transfer to a volumetric flask with washings.
6. Make up to the 250cm3 mark with distilled water to bottom of the meniscus.
7. Shake / invert flask.

Question 12

RP1 - outline how to carry out an acid-base titration.

Answer 12

ave

1. All apparatus rinsed with distilled water and then with the relevant solution.
2. Using a pipette and pipette filler, pipette 25cm3 of NaOH into a conical flask
3. Using a funnel, fill burette with acid and then remove funnel
4. Allow a small quantity of the acid to flow through the burette to ensure jet space is filled
5. Add 2-3 drops of phenolphthalein (or another suitable indicator) to the conical flask place it on a white tile
6. Add acid from burette into conical flask, swirling mixture during addition, until indicator changes colour (pink —> c

Question 13

RP1 - what happens if the jet space is not filled when filling burette up?

Answer 13

it will lead to errors if it then fills during the titration, leading to a larger than expected titre reading

initial reading on burette.- If jet space not filled, titre value higher than volume actually added.

Question 14

RP1 - why do you need to remove the funnel after filling up the burette?

Answer 14

some drops may fall through the titration after the initial reading has been taken meaning a lower volume is recorded than used.

Question 15

RP1 - how would you know if your results are reliable in a titration?

Answer 15

you have got concordant results

Question 16

RP1 - using the same mass of solid, how would you reduce the percentage uncertainty of a titration?

Answer 16

use a lower concentration of NaOH so that the titre value is larger

Question 17

RP1 - what is the general way to reduce the uncertainty in a burette reading

Answer 17

the volume needs to be larger

Question 18

RP1 - 3 ways to make titre volume larger to reduce uncertainty in a burette reading

Answer 18

-increasing the volume of the substance in the conical flask
-increasing concentration of the substance in the conical flask
-decreasing the concentration of the substance in the burette

Question 19

RP1 - why is a conical flask used in a titration?

Answer 19

can be swirled without losing any solution

Question 20

RP 1 - why is swirling necessary?

Answer 20

to ensure solutions completely mix and react

Question 21

RP1 - why does rinsing the flask with water not affect the end point?

Answer 21

water does not react so does not affect the number of moles present

Question 22

RP1 - what are concordant results?

Answer 22

within 0.1cm3 of each other

Question 23

RP1 - how to reduce uncertainties when measuring the mass for a volumetric solution

Answer 23

Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid.
● Weighing sample before and after addition and then calculating difference (weighing by difference) will ensure a more accurate measurement of the mass added.

Question 24

RP1 - why is a pipette used instead of a measuring cylinder?

Answer 24

to accurately measure a precise, fixed volume of liquid and it has a smaller uncertainty

Question 25

why is a burette used

Answer 25

it is accurate

Question 26

RP1 - what is the burette rinsed out with?

Answer 26

the substance that will be put in it

Question 27

RP1 - why is the burette rinsed with the solution that is going in it?

Answer 27

if it is not rinsed out the acid or alkali added may be diluted by residual water in the burette or may react with substances left from a previous titration. This would lead to the concentration of the substance being lowered and a larger titre being delivered

Question 28

RP1 - what happens to titre value if the burette is not rinsed with the solution?

Answer 28

the concentration of the substance would be lowered so the titre would be larger

Question 29

RP1 - why is only a few drops of indicator added?

Answer 29

if too much is added it affects the titre result

Question 30

RP1 - what colour is phenolphthalein in alkali?

Answer 30

pink

Question 31

RP1 - what colour is phenolphthalein in acid?

Answer 31

colourless

Question 32

RP1 - what is a standard solution?

Answer 32

a solution of known concentration

Question 33

RP1 - why are acid-base indicators used?

Answer 33

to detect when a reaction is completed usually by a colour change

Question 34

RP1 - why do you need to do a rough titration first?

Answer 34

as an approximation of where the end point will be so on the next one you know when to add dropwise

Question 35

RP1 - what colour is methyl orange in acid?

Answer 35

red

Question 36

RP1 - what colour is methyl orange in alkali?

Answer 36

yellow

Question 37

why place conical flask on a white tile?

Answer 37

place it on a white tile - can see colour change better.

Question 38

why do you Touch surface of solution with tip of pipette when measuring base volume?

Answer 38

to ensure correct quantity is transferred.