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I assume you're referring to calculations using equilibrium constants. If the equilibrium constant you are using is less than 10-3, you can usually assume that subtracting x does not change a constant very much. However, this does not mean that x is zero. For example, if given 10-5 = x2/(0.15-x), you can assume the denominator is 0.15, but the numerator is still x2.
When the K is less than 10^-3 you can assume that something like (.15-x) in the denominator will become just .15 since the x is so small it does practically nothing to that amount. But in the numerator you will still have an x present like x^2.
Approximation of x is done if K < 10^-3. That means you can take out the x in the denominator (reactants) because it won't have a significant effect on the value you'll obtain at the end. If unsure, you can confirm the approximation when x < 5% of the initial concentration of the molecule asked to be calculated.
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