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NSWChemistryQuick questions

Module 5: Equilibrium and Acid Reactions

Quick questions on pH and pOH calculations explained: HSC Chemistry Module 5

6short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What are significant figures for logs?
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Only the digits after the decimal point in a log are significant. A [H+][H^+] of 1.64×10−31.64 \times 10^{-3} (3 sig fig) gives pH = 2.79 (2 decimal places).
What are wrong sig figs?
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For pH, the number of decimal places equals the sig figs of the concentration. pH = 2.79 (3 sig fig), not 2.794 or 2.8.
What are dilution errors?
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When diluting a strong acid 10-fold, pH increases by 1 (more dilute, less acidic). When diluting a weak acid 10-fold, pH increases by about 0.5 because dissociation increases as concentration falls.
What is q1?
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State the relationship between pH, pOH, [H+][H^+] and [OH−][OH^-] at 25 degrees C. [2 marks]
What is q2?
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Calculate the pH of a 0.020 mol L−1^{-1} solution of Ba(OH)2Ba(OH)_2, a strong diprotic base. [3 marks]
What is q3?
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A 0.10 mol L−1^{-1} solution of formic acid (HCOOH, Ka=1.8×10−4K_a = 1.8 \times 10^{-4}) is prepared. (a) Write the equilibrium expression. (b) Calculate [H+][H^+] using the 5 percent approximation.

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