Need to do analysis of results for core practical 2 Titration Calculations And Errors Objective: to complete titration calculations and know errors that we may have in titrations Outcomes: Be able to calculate amounts of substances (in mol) in reactions involving volume of solutions and concentration
These calculations may involve reactants and/or products. Be able to calculate solution concentrations, in mol dm-3 and g dm-3, including simple acid-base titrations using a range of acids, alkalis and indicators Be able to: i) calculate measurement uncertainties and measurement errors in experimental results ii) comment on sources of error in experimental procedures Understand how to minimise the percentage error and percentage uncertainty in experiments involving measurements
Objectives can carry out an acid-base titration accurately and safely (C) can identify potential errors and mistakes in the titration procedure (B) can predict the effect of mistakes on the true value of the titre (A) Homework
Write An idiots guide to titration Highlight the key parts of the procedure, include references to how you ensure accuracy and reliability. Include how you know if your results accurate Include how know if your results are reliable Make sure you point out potential mistakes and how to avoid them! Processing the titration data
Equation for reaction Amount of standard solution (in moles) Amount of unknown solution (in moles) Concentration of unknown solution (in mol dm-3) Titration calculations examples What is the concentration of an NaOH solution if 25.0 cm3 is neutralized by 23.4 cm3 0.998 mol dm-3 HCl solution?
1. Calculate no. moles HCl: 2. Determine ratio of NaOH to HCl: moles = (conc. volume) / 1000 = (0.998 23.4) /
1000 = 0.0234 NaOH + HCl NaCl + H2O ratio NaOH:NaCl = 1:1 3. Calculate no. moles of NaOH:
= 0.936 mol dm-3 Examples from AQA Analysis of core practical 2 Pg 10, Q5 paper 1 26th May 2016 Example 1
25.0 cm3 of sodium hydroxide solution is exactly neutralised by 27.7 cm3 of 0.20 mol dm-3 hydrochloric acid. Calculate the concentration of the sodium hydroxide solution Example 2 20.0 cm3 of sodium hydroxide solution is exactly neutralised by 15.5 cm3 of 0.108 mol dm-3 sulfuric acid. Calculate the concentration of the sodium hydroxide solution Example 3
25.0 cm3 of an unknown acid, HA, is exactly neutralised by 15.0 cm3 of 0.50 mol dm-3 sodium hydroxide. Calculate the concentration of the acid solution, HA. Challenge Question The concentration of HA in g dm-3 is 10.95 g dm-3 . Calculate the molar mass of HA and use your answer to suggest its identity.
Errors and Mistakes - Starter What is the difference between an error and a mistake? What mistakes might happen in a titration?
leaving funnel in top of burette not getting rid of air bubbles in burette going past the end point not reading burette at eye level Deduce the effect each of these mistakes would have on you final titre Errors can be systematic or random
Reliability and Accuracy How do you know if your results are reliable? How do you know if your results are accurate? Uncertainty is the amount of error your measurements might have Any measurements you make will have uncertainty in them due to the
limits of the sensitivity of the equipment used E.g. a burette has marks every 0.1 cm3 You should be able to tell which mark the levels closet to, so you wont be more than 0.05 cm3 out. So the uncertainty of a reading from burette is the maximum error you could have so thats +/- 0.05 cm3 Percentage Uncertainty
Percentage uncertainty = uncertainty x 100 reading Combined total uncertainty = total uncertainty x 100 difference in readings Total Uncertainty
In titrations heres how you find the total uncertainty in the final result: Find the percentage uncertainty for each bit of equipment Add the individual percentage uncertainties together. This gives the percentage uncertainty in the final result. Use this to work out the actual total uncertainty in the final result You can minimise the percentage uncertainty
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