| Date | May 2016 | Marks available | 3 | Reference code | 16M.3.sl.TZ0.2 |
| Level | SL | Paper | 3 | Time zone | TZ0 |
| Command term | Justify, Outline, and State | Question number | 2 | Adapted from | N/A |
Question
A student wished to determine the concentration of a solution of sodium hydroxide by titrating it against a 0.100moldm−3 aqueous solution of hydrochloric acid.
4.00g of sodium hydroxide pellets were used to make 1.00dm3 aqueous solution.
20.0cm3 samples of the sodium hydroxide solution were titrated using bromothymol blue as the indicator.
Outline, giving your reasons, how you would carefully prepare the 1.00dm3 aqueous solution from the 4.00g sodium hydroxide pellets.
(i) State the colour change of the indicator that the student would see during his titration using section 22 of the data booklet.
(ii) The student added the acid too quickly. Outline, giving your reason, how this could have affected the calculated concentration.
Suggest why, despite preparing the solution and performing the titrations very carefully, widely different results were obtained.
Markscheme
Key Procedural Steps:
use volumetric flask
mix the solution
fill up to line/mark/«bottom of» meniscus/1 dm3 «with deionized/distilled water»
Key Technique Aspects:
use balance that reads to two decimal places/use analytical balance/use balance of high precision
mix pellets in beaker with deionized/distilled water «and stir with glass rod to dissolve»
use a funnel «and glass-rod» to avoid loss of solution
need to rinse «the beaker, funnel and glass rod» and transfer washings to the «volumetric» flask
Safety Precautions:
NaOH corrosive/reacts with water exothermically
keep NaOH in dessicator
let the solution cool
Two marks may be awarded from two different categories or from within one category.
Do not accept “use of a funnel to transfer the solid”.
Do not accept “keep volumetric flask in cold water/ice”.
(i) blue to green/yellow
(ii) equivalence point has been exceeded
OR
greater volume of/too much acid has been added
«calculated» concentration increased
Accept “end-point” for “equivalence point”.
colour difficult to detect
OR
using different HCl standards
OR
no significant figures used in subsequent calculation
OR
incorrect method of calculation
Accept any valid hypothesis.
Do not accept any mistakes associated with techniques (based on stem of question) eg. parallax error, not rinsing glassware, etc.
Do not accept “HCl was not standardized”.
Accept “reaction of NaOH with CO2 «from air»”.
Accept “NaOH hygroscopic/absorbs moisture/H2O «from the air/atmosphere»”.
Accept “impurities in NaOH”.
Accept "temperature changes during experiment".
Ignore a general reference to random errors.
