Learner Resource 4 There are many assumptions that we make when doing pH calculations of acids, bases and buffers. It is essential that you understand and remember them. Task 1: Read the assumptions and tick whether they are true or false. Calculation assumption 1 When calculating the pH of a weak acid we assume that [H+] [A–] 2 When calculating the pH of a strong base we assume that [H+] = [OH–] 3 When calculating the pH of pure water we assume that [H+] = [OH–] 4 When calculating the pH of a buffer we assume that the value given for the concentration of the salt [A–]. 5 When calculating the pH of a strong acid we assume that the acid has fully dissociated. 6 When calculating the pH of a weak acid we assume that the acid has fully dissociated. 7 When calculating the pH of a buffer we assume that [H+] [A–] 8 When calculating the pH of pure water we assume that Kw = [H+]2 9 When calculating the pH of a strong base we assume that [H+] [OH–] 10 When calculating the pH of a strong acid, we assume that the concentration of the acid [H+]. 11 In buffers made from an excess of weak acid we assume that [HA] [excess acid calculated], i.e. calculated using amount of acid – amount of base 12 In buffers made from an excess of weak acid we assume that [A–] [OH–], i.e. calculated from the amount of base given in the question. Version 1 pH and Buffers 1 True False © OCR 2016 Task 2: Write the true calculation assumptions into the correct boxes below according to which type of calculation they relate to. Weak acid Strong acid Strong base Water Buffer Task 3: Now look at the statements you marked as false in Task 1. First check if there is an equivalent true statement. If not, modify each statement to make it true and add it to the appropriate box in Task 2. There are only two to do. Task 4: Learn these assumptions off by heart. Version 1 pH and Buffers 2 © OCR 2016 OCR Resources: the small print OCR’s resources are provided to support the teaching of OCR specifications, but in no way constitute an endorsed teaching method that is required by the Board, and the decision to use them lies with the individual teacher. Whilst every effort is made to ensure the accuracy of the content, OCR cannot be held responsible for any errors or omissions within these resources. © OCR 2016 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work. Please get in touch if you want to discuss the accessibility of resources we offer to support delivery of our qualifications: resources.feedback@ocr.org.uk Version 1 pH and Buffers 3 © OCR 2016