Lesson 5a: deliquescence of sodium nitrate |
1st Calculation |
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Note: the above should be entered on the
variable "relative humidity, or total water" parametric calculations page of
Model III |
First, plot a graph of moles of liquid phase water against relative humidity:
1st Graph: select the variables and enter the options as given below. | |
X Variable: "relative humidity" Range: leave blank Scale: linear (the default) |
Y Variable: "moles of H2O(aq)" Range: leave blank Scale: linear (the default) |
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame. |
Notice that the transition from the dry salt to a solution containing 5.25 moles of liquid water occurs at about 0.7375 (73.75%) relative humidity: this is the deliquescence point of sodium nitrate at 298.15 K. The salt thus remains a dry solid to a significantly higher relative humidity than does NH4NO3 (61 %) which was studied in Lesson 2.
Next we determine the composition of the aqueous solution of
NaNO3 at the deliquescence point by plotting
the molality of Na+(aq) against relative
humidity:
2nd Graph: select the variables and enter the options as given below. | |
X Variable: "relative humidity" Range: leave blank Scale: linear (the default) |
Y Variable: "mNa+(aq)" Range: leave blank Scale: linear (the default) |
Click on the "Draw the Graph" button at the end of the page, and the plot will appear in the right frame. |
At low relative humidities the molality of Na+ (aq) is zero, which corresponds to the dry particle. At the deliquescence relative humidity of 73.75% the particle takes up water, giving a molality of about 10.6 mol kg-1 in the liquid droplet. This value could also have been calculated from the moles of liquid water per mole of salt as follows: a saturated solution contains 5.25 moles or 5.25 × 0.0180152 = 0.09458 kg of water per mole of salt. This corresponds to a molality of 1 / 0.09458 = 10.6 mol kg-1.
The 10.6 kg-1 molality of the saturated solution compares with the value of about 26.5 mol kg-1 for NH4NO3 which deliquesces at 61% relative humidity. This is consistent with the general observation that solutions become more concentrated as relative humidity is decreased.
Proceed to Lesson 5b to learn how mixing different salts affects their deliquescence behaviour, or return to the main page for this lesson.