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I noticed that when calculating K/Kc in questions 11.3 and 11.9 (6th edition), the solutions had either brackets or parentheses. Is there a difference between the two? How would I know which one to use?
Brackets are used when referring to concentration amounts. However, once you start calculating K or Kc, although we use concentration amounts to obtain the equilibrium constant, using concentration is still an approximation. We are actually trying to find equilibrium constant using chemical activity, which has no units. So when you use concentration amounts to find K, you are assuming that the molecules are acting under ideal conditions, and use the numbers as if you are finding chemical activity, and this is when you will use parantheses with no units for the numbers inside, since again chemical activity has no units.
Another way to think about it is that brackets are used when you're referring to a concentration and parentheses are used when you substitute in the actual numerical value of the concentration into the K equation. For example, if one of the compounds in your chemical reaction is NH3 and its concentration is 0.30 mol/L, then in the K equation, you'd write either write [NH3] or (0.30).
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