Flashcards · H432 · Topic 1

Development of practical skills in chemistry flashcards

27 cards on Topic 1, subtopics 1.1.1 to 1.2.2. Pick a subtopic or study them all.

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Independent, dependent and controlled variables: Independent: the one you deliberately change. Dependent: the one you measure. Controlled: any other variable that could affect the result, kept constant.
Steps in writing a practical plan: Name the independent and dependent variables with units; choose apparatus and say why; choose quantities to fit the apparatus; name the controlled variables and how each is controlled; describe measurements and repeats; say how results are processed.
A plan expects about 80 cm³ of gas in a 50 cm³ gas syringe. What is wrong, and how is it fixed?: The reading would overflow the apparatus, so the method is not appropriate. Use a 100 cm³ syringe or less reactant.
Three checks that a method is appropriate: Does it measure the right quantity? Are readings within range and large enough for a small percentage uncertainty? Is anything lost, gained or side-reacting?

How should column headings and readings appear in a results table?: Headings as quantity / unit, with no units in the body. Independent variable in the first column. The same number of decimal places down a column, matching the resolution.
To how many decimal places is a burette reading recorded?: Two decimal places, ending in 0 or 5 (e.g. 23.45 cm³).
Units of Kc = [C]² ÷ ([A][B]³): (mol dm⁻³)² ÷ (mol dm⁻³)⁴ = (mol dm⁻³)⁻² = dm⁶ mol⁻².
Why rinse a burette with the titrant and a pipette with the solution it delivers?: So that any water left inside does not dilute the solution, which would change its concentration.

1.37 g NaCl (M = 58.44) made up to 250.0 cm³. Concentration to the correct significant figures?: 1.37 ÷ 58.44 ÷ 0.2500 = 0.09377…; the mass has 3 s.f., so 0.0938 mol dm⁻³.
How do you find the gradient of a curve at a point?: Draw a tangent at that point, then gradient = Δy ÷ Δx using two well-separated points on the tangent.
Graph-plotting rules: Independent variable on the x-axis; axes labelled quantity / unit; points fill more than half the grid; scale steps of 1, 2 or 5; small crosses; one thin best-fit line or smooth curve.
Finding an intercept when the axis does not start at zero: Use c = y − mx with a point on the line; do not read it off the axis.

Accurate vs precise: Accurate: close to the true or accepted value. Precise: repeated values are close to one another.
Random vs systematic error: Random: unpredictable scatter either side; reduced by repeating and averaging. Systematic: the same direction every time; repeats do not remove it.
Percentage uncertainty of a temperature change of 6.0 °C read on a thermometer to ±0.5 °C: Two readings, so ±1.0 °C. 1.0 ÷ 6.0 × 100 = 17%.
Percentage uncertainty for a quantity calculated by multiplying or dividing measurements: Add the percentage uncertainties of the measured quantities.
Percentage error is larger than the total percentage uncertainty. What does that show?: There is a systematic limitation in the method, not just measurement uncertainty. Name it, its direction of effect, and a matching improvement.

What does the Practical Endorsement require?: A minimum of 12 assessed practical activities covering PAG1 to PAG12, with every 1.2.1 skill and 1.2.2 technique shown. Teacher assessed and reported separately as Pass or Not Classified.
Hazard vs risk: Hazard: something that could cause harm. Risk: the chance it does cause harm in your method. The control measure must match the named hazard.
What are the five CPAC areas?: Follows written procedures; applies investigative approaches; works safely; makes and records observations; researches, references and reports.
What should a source citation include?: Author, title, date, and for a website the URL and access date.

How do you make up a standard solution?: Weigh the solid by difference, dissolve in distilled water in a beaker, transfer to a volumetric flask with washings, make up so the bottom of the meniscus is on the mark at eye level, stopper and invert.
Concordant titres, and which titres to average: Titres within 0.10 cm³ of each other. Average only the concordant titres, never the rough titre.
Distillation: where does the thermometer bulb go?: Level with the side arm of the still head, so it reads the temperature of the vapour being collected.
How do you recrystallise an impure solid?: Dissolve in the minimum hot solvent, hot-filter (fluted paper) if needed, cool slowly then in ice, filter under reduced pressure, wash with ice-cold solvent, dry.
Melting point of a pure vs an impure solid: Pure: sharp, at the known value. Impure: lower and over a range.
Why heat flammable organic liquids with a water bath or heating mantle?: There is no naked flame to ignite the vapour.