Calculation: If you have a pH of 5.5 for a weak acid with a pKa of 4.76, then is there more A- or more HA in the solution? Expla
in why in words using your knowledge of positive or negative log numbers.
2 answers:
Answer:
lets set the ratio -A/HA as R:
pH = pKa + log(R,10) => pKa + log10(R)
pH = 5.5
pKa = 4.76
R => 10^(pH - 4.76)
10^(pH - 4.76) => 5.4954
Given R (-A/HA) a number bigger than 1, then the concentration of -A is bigger than HA
Explanation:
Answer:
There is more A⁻ than HA in the solution
Explanation:
The equation for the ionization of a weak acid is
HA + H₂O ⇌H₃O⁺ + OH⁻
When HA and A⁻ are present in comparable amounts, we can use the Henderson-Hasselbalch equation:
![\begin{array}{rcl}\text{pH} &=& \text{pK}_{\text{a}} + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\5.5 & = & 4.76 + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\0.74 & = & \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\end{array}](https://tex.z-dn.net/?f=%5Cbegin%7Barray%7D%7Brcl%7D%5Ctext%7BpH%7D%20%26%3D%26%20%5Ctext%7BpK%7D_%7B%5Ctext%7Ba%7D%7D%20%2B%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5C%5C5.5%20%26%20%3D%20%26%204.76%20%2B%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5C%5C0.74%20%26%20%3D%20%26%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5Cend%7Barray%7D)
0.74 is a positive number, and a number must be greater than one for its logarithm to be positive. That is,
![\begin{array}{rcl}\dfrac{[\text{A}^{-}]}{\text{[HA]}} & > & 1\\\\\textbf{[A]}^{-} & > & \textbf{[HA]}\\\end{array}](https://tex.z-dn.net/?f=%5Cbegin%7Barray%7D%7Brcl%7D%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%20%26%20%3E%20%26%201%5C%5C%5C%5C%5Ctextbf%7B%5BA%5D%7D%5E%7B-%7D%20%26%20%3E%20%26%20%5Ctextbf%7B%5BHA%5D%7D%5C%5C%5Cend%7Barray%7D)
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