Course

AS Chemistry: Practical Skills and Paper 3 (9701)

Name: AS Chemistry: Practical Skills and Paper 3 (9701)
Price: 1499 BDT

The paper where knowing the chemistry is not enough. Choose the apparatus the method asks for, read it to the right place, and say which measurement limits your answer.

16 lessons1h 22mEnglish

What you'll learn

Choose apparatus from the number of significant figures a method quotes, and reject over-precision as well as under-precision
Read a burette to half a division and explain why a titre carries twice a single reading’s uncertainty
Lay out a results table with units in the headings and consistent decimal places
Distinguish precision from accuracy, and a random error from a systematic one that repeating cannot reveal
Make a standard solution, and state the direction in which each procedural error shifts the result
Justify every step of a titration, exclude a rough titre, and defend the exclusion
Choose an indicator by matching its range to the pH at equivalence, and say why weak against weak needs a meter
Convert a mean titre into a concentration through the balanced equation, with the volumes in the right units
Carry out cation tests a little at a time and in excess, and separate zinc from aluminium with the right reagent
Acidify before an anion test with the acid that does not introduce the ion being sought
Report an observation and an inference separately, and record a negative result as a result
Combine uncertainties correctly: absolutes add on a subtraction, percentages on a division
Identify the measurement that limits a result by ranking percentage uncertainties
Evaluate a method in three parts: the weakness, the change, and the direction the result moves

Requirements

AS stoichiometry: the mole, concentration and balanced equations
AS equilibria: strong and weak acids, and the shape of a pH curve

Course content

16 lessons · 1h 22m

Measurement and apparatus

The wording of a method tells you the glassware. Then how far to read a scale, why a titre carries two uncertainties, and how to make a solution you can trust.

0/3

Choosing the apparatus

The number of figures in the method tells you which glassware to use, and over-precision is as wrong as under-precision.

Preview6m 2.

Reading and recording a scale

Read to half the smallest division, and remember that a titre is a subtraction, so it carries two reading uncertainties.

Preview6m

Making a standard solution

Precision is not accuracy, and a systematic error hides where a random one shows. Then every step of making a solution you can trust.

Titration

Rinse with the solution, not with water. Exclude the rough titre. Match the indicator to the equivalence pH. Then four fixed steps from titre to concentration.

0/3

The titration procedure

Every step has a reason. The rough titre is recorded and excluded, and concordant means agreeing within 0.10 cm³.

Preview7m 2.

Choosing the indicator

The indicator range has to lie inside the vertical section of the curve, which is why weak against weak needs a pH meter.

Preview7m

From titre to concentration

Four fixed steps, and the only one that needs thought is the mole ratio. That is where a factor of two goes missing.

Qualitative analysis

Add the reagent a little and then in excess. Acidify before an anion test so a carbonate cannot interfere. Record what you saw, then what it means.

0/3

Testing for cations

One reagent gives two observations, and only excess ammonia separates zinc from aluminium. Ammonium has no precipitate at all.

Preview7m 2.

Testing for anions and gases

Acidify to destroy carbonate, and never with the acid containing the ion you are seeking. Then the six gas tests in their exact wording.

Preview7m

Identifying an unknown salt

Choose the order that eliminates fastest, record negatives as results, and say "suggests" after one test and "confirms" after two.

Uncertainty, evaluation and planning

The apparatus fixes the absolute uncertainty and you fix the measurement, so bigger measurements are better ones. Then which one limits the answer, and how to improve a method.

0/3

Absolute and percentage uncertainty

Absolute uncertainties add on a subtraction and percentages add on a division. Mixing the two rules is the commonest error.

Preview7m 2.

Which measurement limits the result

Rank by percentage, not by how precise the instrument sounds, and improve only the one that dominates.

Preview7m

Evaluating and improving a method

Name the weakness, the change and the direction the result moves. Repeating fixes a random error and never a systematic one.

About the creator

8bitlesson

Cambridge O Level, A Level and university courses, taught through interactive labs.

Mentora builds the sciences and mathematics as things you can move rather than videos you watch. Cambridge O Level and A Level mathematics, physics, chemistry and accounting, a primary mathematics track, and first-year university computer science. Every lesson works a real exam question end to end.