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Calculating concentrations

Calculating concentrations. The concentration of a solution can be measured in grams per dm 3 (g/dm 3 ) or moles per dm 3 (mol/dm 3 ). The following equation gives concentration in g/dm 3 :. mass dissolved (g). concentration =. volume of solution (dm 3 ).

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Calculating concentrations

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  1. Calculating concentrations The concentration of a solution can be measured in grams per dm3 (g/dm3) or moles per dm3 (mol/dm3). The following equation gives concentration in g/dm3: mass dissolved (g) concentration = volume of solution (dm3) If 1.0g of solid sodium hydroxide is dissolved in 250cm3 of solution, what is the concentration in g/dm3? mass of solid = 1g volume of solution = 250cm3 = 0.25dm3 concentration = 1/0.25 = 4g/dm3

  2. Standard solutions

  3. Calculating concentrations in g/dm3

  4. Calculating concentrations in mol/dm3 To calculate concentration in mol/dm3: mass dissolved (mol) concentration = volume of solution (dm3) The equation for concentration can be put into a formula triangle: m c = v   x

  5. Calculating concentration If 1.0g of solid sodium hydroxide are dissolved in 250cm3 of solution, what is the concentration in mol/dm3? • Convert mass of solid into moles of solid: RAM of sodium hydroxide = 40 moles = mass/RAM = 1/40 = 0.025mol • Calculate concentration: volume of solution = 250cm3 = 0.25dm3 concentration = moles/volume = 0.025/0.25 = 0.1mol/dm3

  6. Calculating concentrations in mol/dm3

  7. Titration Titration is a technique that can be used to accurately analyse the concentrations of substances in solution. safety filler burette stand pipette conical flask beaker Titrations are often carried out by using a neutralization reaction between an acid and an alkali.

  8. How does titration work? Imagine you have a sample of hydrochloric acid and you need to know its concentration. First, measure out a specific amount of the acid, and neutralize it with a standard solution of an alkali. Measuring the amount of alkali that is needed to neutralize the acid will allow you to work out the concentration of the acid. If you measured out 25cm3 of the unknown acid, and found that it was neutralized by 20cm3 of 0.1mol/dm3 alkali, is the acid more or less concentrated than the alkali? The acid is less concentrated than the alkali.

  9. How does titration work?

  10. Titration calculations How are the results of a titration used to calculate the concentration of an unknown acid solution? Titration Attempt 1 Attempt 2 Attempt 3 initial burette reading (cm3) 0.0 0.0 19.9 final burette reading (cm3) 20.0 19.9 40.0 volume of NaOH added (cm3) 20.0 19.9 20.1 Average volume of NaOH = (20.0 + 19.9 + 20.1)/3 = 20.0cm3

  11. Titration calculations Result: 20cm3 of NaOH neutralizes 25cm3 of HCl of unknown concentration. Working: 1. Write a balanced equation for the reaction: HCl + NaOH → NaCl + H2O 2. Calculate the number of moles of alkali: moles = concentration (mol/dm-3) × volume (dm3) = 0.1 × (20.00 / 1000) = 0.002 moles NaOH

  12. Titration calculations 3. The balanced equation shows that one mole of HCl reacts with one mole of NaOH, so 0.002 moles of NaOH will react with 0.002 moles of HCl. 4. Calculate the concentration of the HCl solution in mol/dm3: moles concentration = volume (dm3) = 0.002 / (25 / 1000) = 0.08 mol/dm3

  13. Titration calculations

  14. Titration apparatus

  15. Making accurate measurements

  16. Choosing suitable measuring apparatus

  17. Plotting a pH curve

  18. pH curve for strong alkali and strong acid How would the pH curve look if you started with a strong alkali in the conical flask and added the strong acid to it from the burette? 14 The pH starts off high and steadily decreases as the acid is added. The endpoint is at pH7. pH 7 0 0 5 10 15 20 25 30 35 40 45 50 volume acid added (cm3)

  19. Producing different pH curves

  20. End points The endpoint of a titration is not always at pH 7. Titrations involving a weak acid or alkali can cause the indicator to change colour at a different pH. Alkali Acid pH at endpoint strong strong 7 less than 7 weak strong strong weak more than 7 7 weak weak

  21. pH ranges of indicators

  22. Indicators and pH ranges When choosing an indicator for a titration, it is important to match its pH range (the pH values where it changes colour) to the endpoint of the titration. • Phenolphthalein has a pH range of 8–9. • Methyl orange has a pH range of 3–4. • Litmus has a pH range of 5–8. • Bromothymol blue has a pH range of 6–7. Why is litmus not a particularly good indicator for a titration?

  23. Choosing the right indicator

  24. True or false?

  25. Glossary

  26. Anagrams

  27. Multiple-choice quiz

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