Core Practical 2: Find the concentration of a solution of sodium hydroxide

Question 1

sulfamic acid

Answer 1

can be toxic if injested

Question 2

when filling a burette safely, the top of the burette should be

Answer 2

below eye level

Question 3

the method for making up the standard solution of sulfamic acid, H3NSO3, is

Answer 3

• weigh out 2.5g of sulfamic a acid onto a weighing boat
• add this to a 250cm3 beaker and reweigh the empty weighing boat
• add about 100cm3 of deionised water to the solid acid to dissolve it, then transfer to a 250cm3 volumetric flask, adding the washings from the beaker to the flask and making up to the mark with deionised water
• place the stopper in the flask and invert several times to ensure complete dissolution of acid

Question 4

if 2.5g of H3NSO3 acid is used, what is the concentration of the solution in the volumetric flask?

Answer 4

2.5 / 97.1
= 0.02575mol H3NSO3

0.02575 / 250/1000 = 0.10 mol dm-3

Question 5

the method for properly filling a burette is:

Answer 5

• add some of the acid to the burette and open the tap
• close the tap once all of the acid has run through, then invert the burette and turn it so that the remaining acid coats the sides of the burette
• fill the burette with the acid, ensuring the bottom of the meniscus is at 00.00cm3 when read at eye level

Question 6

the method for finding out the concentration of the NaOH solution of unknown concentration is:

Answer 6

• set up a titration with the sulfamic acid standard solution in the burette and 25cm3 of unknown concentration NaOH in a conical flask
• pipette aliquots of 25cm3 by first rinsing the pipette with water, then with NaOH before actually measuring out the 25cm3 of NaOH and transferring it
• add four drops of methyl orange into the flask and place this beneath the burette on a white tile
• titrate the contents of the flask against the sulfamic acid until the mixture turns from red to yellow
• repeat until 2 concordant results (within 0.20cm3 of each other) have been obtained

Question 7

is phenolphthalein was used, the colour change would be

Answer 7

pink to colourless

Question 8

the method to calculate the concentration of the NaOH is the concentration of the sulfamic solution was 0.10 mol dm-3 is:

Answer 8

• sulfamic acid is a monoprotic acid, so will react with the NaOH in a 1:1 ratio
• mols of H3NSO3 = 0.10 x mean titre/1000
• mols of H3NSO3 = mols NaOH
• concentration of NaOH = mols of NaOH/ 25/1000

Question 9

the equation for calculating percentage uncertainty is:

Answer 9

percentage error x number of measurements / measurement made