Chemistry practical notes

Practical tools Electronic balance - Proximity 0.01g Beam balance - Dependent on calibration of the basis weights Metre rule - Proximity 0.1cm Measuring tape - Proximity 0.1 or 0.5cm Thermometers 1. Mercury 2. Alcohol 3. Digital 4. Data loggers Volumetric flask Fixed volume of or 100 𝑐𝑚 ³ 250 𝑐𝑚 ³ Usage: to acquire an accurate volume of a chemical. Conical flask Usage: to mix chemicals without spillage Measuring cylinder Proximity of 0 . 5 𝑐𝑚 ³ Usage: to measure volume Burette Proximity of 0 . 05 𝑐𝑚 ³ Usage: dispense a specific volume of acid Pipette Fixed volume of , 10 . 0 𝑐𝑚 ³ or 20 . 0 𝑐𝑚 ³ 25 . 0 𝑐𝑚 ³ Usage: acquire an accurate volume of chemical Gas syringe Fixed volume of 100 𝑐𝑚 ³ Usage: measure gas volume Fractionating column Small solid objects (s.a., glass beads) to provide a larger surface area for vapour condensation Separating funnel Filter funnel Filter paper Stop watch Breaker Test tube Boiling tube Water bath The water bath keeps the given solution/mixture at a constant temperature, allowing for more accurate experiments. Calculation D.P. rule Whenever calculating two values that only involve addition and subtraction, the calculated value adopts the least d.p., with a minimum of 0d.p. For s.f., For the final answer, round to 3 s.f. For method calculation, use 4 s.f. To 6 s.f. Graphical analysis

Plot against , based on the given equation and determine the relationship of the graph. 𝑌 𝑋 Proportional relationship When two quantities are directly or inversely proportional to each other. 1. The graph is a straight line 2. Passes through the origin (0,0) Linear relationship When two quantities are linearly related. 1. The graph is a straight line Non-linear relationship Two quantities create a graph that is curved. 𝑌 = 𝑘𝑋 𝑌 = 𝑚𝑋 + 𝐶 𝑌 = 𝑓 ( 𝑋 ) Constant relationship When two quantities that are independent of each other, the graph produces a straight horizontal line. Note: Some graphs are expected to pass through the origin based on the physical relationship between the variables. - For example, if the dependent variable represents the change caused by a limiting reactant, adding 0 volume of the reactant should produce 0 change in the system. Therefore, the graph should pass through the origin . ( 0 , 0 ) 𝑌 = 𝑘 Quantity Algebra, both the numerical magnitude and unit of a quantity are substituted for its symbol in the equation. Number Algebra, only the numerical magnitude of a quantity is substituted for its symbol in the equation, converted to the usual units by applying the appropriate multiplying factor. Errors and uncertainty Systemic error Constant deviations of the readings in one direction from the true value. - Zero error - Reaction time - Wrongly calibrated scale Systematic errors can be reduced by calibration curves and control experiments. Random error Measurements are scattered around a mean value. - Reading a scale about interpolation - Timing oscillations without a reference marker - Taking readings of a quantity that varies with time. - Parallax error. Random error can be mitigated by averaging the values, repeating the experiment or by drawing a graph. Accuracy: how close a measurement is to the actual value - Note: accuracy has a 10% range from the true value. Precision: how close the measurements are to each other, or to a mean value Uncertainty is a range of values in which a measurement can fall. - Uncertainty of a digital equipment is one unit of the smallest scale of the instrument - Uncertainty of an analogue instrument is half the smallest division of the scale. 𝑀 1 = 𝑀 0 ± δ 𝑀 Average of values, σ 𝑚 = ∑ 𝑛 𝑥 𝑛 - σ 𝑚 1 = σ 𝑚 ± 𝑚𝑎𝑥 ( 𝑥 ) − 𝑚𝑖𝑛 ( 𝑥 ) 2 - Note: when calculating the mean of values, it follows the d.p. rule

Absolute uncertainty, δPercentage uncertainty, 𝑃 𝑀 = δ 𝑀 𝑀 0 × 100%Addition or subtraction of uncertainties, ( 𝐴 0 + 𝐵 0 ) ± ( δ 𝐴 + δ 𝐵 )Multiplication or division of uncertainties, 𝐴 0 𝐵 0 ± 𝐴 0 𝐵 0 ( 𝑃 𝐴 + 𝑃 𝐵 )Power or root of uncertainties, 𝐴 0 𝐾 𝐵 0 𝐽 ± 𝐴 0 𝐾 𝐵 0 𝐽 ( | 𝐾 | 𝑃 𝐴 + | 𝐽 | 𝑃 𝐵 )Note: d.p. Of both must be the same. Increasing accuracy When measuring the diameter of an object, take repeated readings at different position across the diameter, finding the average of all the readings. When measuring the time taken, take repeated readings, finding the average of all the readings. Precision of instrument For digital measuring instruments, the measurement uncertainty is typically ±1 in the last displayed digit. Therefore, record all the digits shown by the display. - Example: If a digital micrometre displays 12.3456 mm, record 12.3456 mm ± 0.0001 mm. Do not round it to 12.346 mm. - Includes: Digital multimeter, digital micrometre screw gauge, digital vernier callipers. For analogue measuring instruments with discrete scale divisions, the measurement uncertainty is typically ±½ of the smallest scale division. - Example: A metre rule has the smallest division of 0.1 cm (1 mm), so the uncertainty is ±0.05 cm (±0.5 mm). Measurements are therefore estimated to the nearest 0.05 cm. - Includes: voltmeter, ammeter, thermometer, ruler, vernier callipers, micrometre, screw gauge. Factors that impact yield 1. Incomplete reaction → some reactants remain unreacted. 2. Side reactions → reactants form unwanted products. 3. Product lost during transfer → some sticks to apparatus. 4. Product lost during filtration → some passes through or remains on the apparatus. 5. Product lost during washing → some soluble product dissolves away. 6. Product lost during crystallisation → some remains dissolved in the solution. 7. Product lost during heating → decomposition may occur if overheated. 8. Accidental losses → crystals/precipitate may be spilled or left behind. Factors that impact purity 1. Unreacted reactants remain in the product. 2. Side products/by-products are present. 3. Impurities in the starting materials. 4. Insufficient washing → soluble impurities remain. 5. Incomplete filtration → unwanted solid remains. 6. Incomplete separation → another substance remains mixed with the product. 7. Insufficient drying → water/solvent remains in the product. Experiments 1.Separation techniques Magnetic

Experimental procedure 1. Slowly pass a magnet through the sample, sweeping the entire mixture. 2. The magnetic materials would be attracted to the magnet, and can be collected as such; the remaining non-magnetic substances would remain in the container. 3. After the first pass, mix the remaining material and pass the magnet through again. - Repeated separation is particularly useful when the magnetic particles are mixed with a lot of non-magnetic material. Sieving Experimental procedure 1. Place the mixture on a sieve. 2. Rapidly shake the sieve, moving it back and forth, for a while. 3. The finer particles would be sieved out of the mixture; the larger particles would remain in the sieve. Filtration