Ph Wert berechnen?
25 g Natriumhydrogensulfit werden in 500 ml Wasser gelöst. Was ist der ph wert ?
Ks1= 1,23 ×10^(-2) Ks2=6,61 × 10^(-8)
Taschenrechner sind in der Prüfung nicht zugelassen.
Mein Ansatz war zunächst die Konzentration zu berechnen. Stoffmenge ist ungefähr 0.25 ; Konzentration dann ca. 0.5.
Anschließend wollte ich das in die folgende Gleichung einsetzen : pH =0.5 (pks -log (c))
Ich komme dann aber nicht auf die richtige Antwort ( müsste 4.55 sein) wieso ? außerdem bin ich immer verwirrt welcher ks Wert nun der “richtige” ist.
(1 votes)
Loading...
25 g per 500 ml is 50 g per liter at an estimated molar mass of about 100 g/mol is approx. 0.5 mol/l, i.e. a relatively concentrated solution. The pH of a NaHSO3 solution is for sufficiently large concentrations c>0.001 mol/l but approximately concentration-independent c(H3O+)=√K1K2 or pH= −1⁄2lg(K1⋅K2)=1⁄2(pK1+pK2)=4.54
This is how it looks decently calculated:
The lion fraction of the sulfur is present as HSO3 ̄ (violet); SO32 ̄ (blue) is added only at concentrations below 0.001 mol/l; in this concentration range, sulfite is approximately like a weak acid. However, at high concentrations, the pH is constant as is customary for ampholytes. The pH is black, its first derivative white, and the x axis indicates the concentration logarithmically (c=10 ̄x mol/l).
How you calculate all this without a calculator is your thing — I have no idea why you should put such a boundary condition. Fiel Fergniegen!
P.S.: Ignore the messenger stem.
So I really didn’t know that much about it, but I once asked chatgpt and that said:
“It seems that you have understood correctly the approach to calculating the pH value. It could be that your approach has caused confusion with the pKs values. The correct pKs value for sodium hydrogen sulfite (NaHSO3) is crucial.
The equation for the pH is:
\[ \text{pH} = \frac{1}{2}(\text{pKs1} – \log_{10}(c_{\text{NaHSO3}}})]
Make sure you use the appropriate pKs for sodium hydrogen sulfite. The correct pKs value should be used to perform the \( \text{pH} \) calculation.
If your result still does not match the expected response, it could also be rounding errors or another calculation error. Check your calculation operations and use all given values accurately.” – Quote Chatgpt
Source: ChatGPT
Maybe it will help you? Much success still and I hope you will find an answer that will help you:-)