Making Standard Solutions

Question 1

what is a standard solution

Answer 1

a solution whose concentration is accurately known

Question 2

what is the easiest way to prepare a standard solution

Answer 2

• to dissolve a known mass of a substance into water

- to have a known volume of the solution

Question 3

what is the name of the substance that is dissolved into the water

Answer 3

primary standards

Question 4

what should primary standards ideally be in terms of their level of reactivity

Answer 4

• they should be chemically stable
• not absorb water from the atmosphere
• should quickly react with other substances when used in titrations

Question 5

what should primary standards ideally be in terms of availability

Answer 5

they should be available in a high degree of purity

Question 6

what should primary standards ideally be in terms of their solubility and molar mass

Answer 6

• they should be soluble in water

- and should have a high molar mass as solids

Question 7

what are examples of two solutions that are used in titrations despite not being primary standard substances

Answer 7

• hydrochloric acid

- sodium hydroxide

Question 8

why is hydrochloric acid not considered a primary standard

Answer 8

• because it only exists in gaseous and aqueous states

- meaning it cant be a solid even though it has a high molar mass

Question 9

why is sodium hydroxide not considered a primary standard despite being able to be a solid

Answer 9

• it absorbs water vapour and carbon dioxide from the atmosphere
• meaning it isnt chemically stable

Question 10

what does it mean for a substance to be chemically stable

Answer 10

it doesnt decompose or react with substances in the air

Question 11

why is weighing sample of sodium hydroxide solution a problem despite it being able to be accurately weighed

Answer 11

• because it can react with water and CO2 in the atmosphere
• you cant be certain about what is being weighed
• as there are unknown masses of water and CO2 in it

Question 12

what is sulfamic acid

Answer 12

• a readily available primary standard for use in acid-base titrations
• that has the necessary characteristics

Question 13

if sulfamic acid has the formula NH2SO3H, what is its molar mass

Answer 13

97gmol-1

Question 14

you have 250cm3 of 0.1M of sulfamic acid. how would you calculate its mass

Answer 14

• moles = concentration x volume
• 0.25 x 0.1 = 0.025 mols
• mass = moles x Mr
• 0.025 x 97 = 2.4g

Question 15

what apparatus do you need for making a standard solution of sulfamic acid

Answer 15

• safely glasses and a lab coat
• an accurate balance (+- 0.001g)
• weighing bottle
• spatula
• 250cm^3 beaker
• 250cm^3 volumetric flask
• wash bottle with deionised water
• small funnel
• glass stirring rod

Question 16

now that you have calculated the rough mass of sulfamic acid, what is the first step in the experiment

Answer 16

• adding between 2.3 and 2.5g of sulfamic acid to the weighing bottle
• in order to weigh accurately

Question 17

what do you after you have weighed the weighing bottle with the sulfamic acid

Answer 17

• transfer as much as possible of the acid into a beaker

- then reweigh the weighing bottle

Question 18

what do you then add to the beaker

Answer 18

• 100cm^3 of deionised water

- stir with the rod until all the sulfamic acid has dissolved

Question 19

what do you do the stirring rod after you have used it

Answer 19

• wash traces of the solution on the rod into the beaker

- using deionised water

Question 20

how would you then transfer the solution in the beaker into the volumetric flask

Answer 20

by placing a funnel into the neck of the volumetric flask

Question 21

what do you do to the beaker after this

Answer 21

• you wash the beaker with the wash bottle

- and transfer the rinsings into the volumetrc flask

Question 22

why is even more deionised water added to the flask after all of this

Answer 22

to make the graduation mark exactly on the flask

Question 23

how would you then make a uniform solution in the flask

Answer 23

by putting a stopper on the flask several times and inverting it