A student who is performing this experiment pours an 8.50 mL sample of the satur
ID: 976161 • Letter: A
Question
A student who is performing this experiment pours an 8.50 mL sample of the saturated borax solution into a 10 mL graduated cylinder after the borax solution had cooled to a certain temperature T. The student rinses the sample into a small beaker using distilled water, and then titrates the solution with a 0.500 M HCl solution. 12.00 mL of the HCl solution is needed to reach the endpoint of the titration. Calculate the value of K_sp for borax at temperature T. (Answer: 0.176 Show your work.) Here is a suggested procedure for doing this calculation: Calculate the number of moles of HCl that were added during the titration. Use reaction (2) in the lab manual to relate the number of moles of HCl to the number of moles of tetraborate ion in the 8.50 mL sample. Calculate the concentration of tetraborate ions in the 8.50 mL sample. Use reaction (1) in the lab manual to relate the concentration of tetraborate ions to the concentration of sodium ions. Use the concentrations of tetraborate ions and sodium ions to calculate the equilibrium constant (K_sp) at temperature T for reaction (1) in the lab manual. Do you expect Delta Sdegree for the dissolution of borax to be a positive or negative number? Explain your reasoning.Explanation / Answer
The balance reaction is as follow:
Na2B4O7*10H2O(borax) + 2HCl ---------> 2NaCl + 4H3BO3 + 5H2O
the moles of HCl = 0.5 * 0.012 = 0.006 moles
moles of tetraborate = 0.0006 / 2 = 0.003 moles
Concentration = 0.003 / 0.0085 = 0.35 M
The relation between borax and sodium is 1:2 so:
Concentration Na = 0.035 * 2 = 0.70 M
Ksp = [Na]2[Borax]
Ksp = 0.72 * 0.35
Ksp = 0.1715
It shoudl be positive, because if we plot lnKsp vs 1/T, we will get a linear plot of this form:
lnKsp = dS/R - dH/R(1/T)
The y intercept will give us the value to calculate dS, and it should be positive.
Hope this helps
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