Question
From the dissociation constant of HNO3 at 25°C in the box, find the percent dissociated in 0.100 M HNO3.
Concentrated HNO3 Is Only Slightly Dissociated2 Strong acids in dilute solution are essentially completely dissociated. As concentration increases, the degree of dissociation decreases. The figure shows a Raman spectrum of solutions of nitric acid of increasing concentration. The spectrum measures scattering of light whose energy corresponds to vibrational energies of molecules. The sharp signal at 1 049 cm in the spectrum of 5.1 M NaNO3 is characteristic of free NO nion A 10.0 M HNO3 solution has a strong NO3 signal at 1049 cm from dissociated acid. Bands denoted by asterisks arise from undisso- ciated HNO3. As concentration increases, the 1 049 cm signal disap- pears and signals attributed to undissociated HNO3 increase. The graph shows the fraction of dissociation deduced from spectroscopic mea- surements. It is instructive to realize that, in 20 M HNO2, there are fewer H2O molecules than there are molecules of HNO3. Dissociation decreases because there is not enough solvent to stabilize the free ions Theoretical studies indicate that dilute HNO3 at a water-air 98.6 wt % HNO3 interface is also a weak acid because there are not enough H20 molecules to solvate the free ions. This finding has implications for atmospheric chemistry at the surface of microscopic droplets in clouds 23.4 M HNO3 21.8 M HNO2 1.0 0.8 0.6 0.4 0.2 0.0 HNO3 19.7 M HNO 15.7 M HNO Raman NMR 10.0 M HNO 48.3 wt % HNO3 20 Formal concentration (M) 5.1 M NaNO3 Temperature (C) Acid dissociation constant (Ka 1 360 cm1 1 049 cmm 720 cm Raman spectrum of aqueous HNO at 25°C. Signals at 1 360, 1 049, and 720 cm1 arise from NO anion. Signals denoted by asterisks are from undissociated HNO3 The wavenumber unit, cm-1, is 1/wavelength 0 25 50 46.8 26.8 14.9
Explanation / Answer
HNO3->H+NO3
0.1 0 0
-X X X
IN THE END:= Ka=X*X/0.1-X
SOLVE THE QUADRATIC EQUATION
X= 0.09
percent dissociated
0.09/0.1 * 100 = 90%
