Lab Manual- Chapter 5

Purpose To use gravimetric analysis to determine the percent by mass of water in a hydrated salt. The mass of the hydrated salt is measured, the sample is heated, and then the mass of remaining sample is measured again.
Hydrated A number of water molecules are chemically bound to the ions of the salt in its crystalline structure.
Waters of Crystallization Water molecules that are within the salts.The number of moles of water per mole of salt is usually a constant
Anhydrous Salt The water molecules are bound to the ions weakly, so heat can move them
Efflorescent Hydrated salts that spontaneously lose water molecules to the atmosphere
Deliquescent Salts that readily absorb water
Gravimetric Analysis An analytical method that relies almost exclusively for the analysis
Procedure 1. obtain sample, place in crucible2. heat the sample3. Measure the new sample
Calcium Chloride, a deliquescent salt, is used as a desiccator salt to maintain a dry environment. Explain. Deliquescent salts absorb water
What is the purpose of firing the crucible? To rid the crucible or sample of any moisture that might be present.
A 2.914 g sample of gypsum, a hydrated salt of calcium sulfate (CaSO4) is heated at a temperature greater than 170C in a crucible until a constant mass is reaced. The mass of the anhydrous CaSO4 salt is 2.304 g. Calculate the percent by mass of water in the hydrated calcium sulfate. Hydrated salt – Anhydrous salt / Hydrated Salt2.914 – 2.304 = .610.610/2.914 = .209 –> 20.9%
The gravimetric analysis of this experiment is meant to be quantitative; therefore, all precautions shuld be made to minimize errors in the analysis. The crucible and lid are handled with tongs. How does this maintain the integrity of the analysis? Using tongs is needed to keep oils from our fingers off of the crucible. Oil can cause the crucible to gain reported mass.
Mass measurements of the crucible, lid, and sample are performed only at room temperature, Why is this necessary for a gravimetric analysis? Scales are calibrated at room temperature, need to make sure sample is at a state similar to when the initial mass was recorded
Mass of hydrated salt Mass of fired crucible/lid/salt – Mass of fired crucible/lid41.600 – 39.030 = [2.570]
Mass of Anhydrous Salt Mass of fired crucible/lid/anhydrous salt – Mass of fired crucible/lid41.600 – 40.340 = [1.260]
Mass of Water Lost Mass of hydrated salt – mass of anhydrous salt OR(Mass of salt/crucible/lid- mass of crucible/lid) – (Mass of anhydrous salt/crucible/lid – mass of crucible/lid)2.570 – 1.26 = [1.31]
Percent by Mass of Volatile Water in Hydrated Salt (Mass of hydrated salt – mass of anhydrous salt) / mass of hydrated salt(2.57 – 1.26)/ 2.57 = .50 –> 50%
During the cooling of the fired crucible, water vapor condensed on the crucible wall before its mass measurement. The condensation did not occur following thermal decomposition of the hydrated salt in Part B. Will the percent water in the hydrated salt be reported as being too high, too low, or unaffected? If water vapor condensed on the wall of the crucible before its mass measurement, the recorded percentage of water will be too high. This is due to the fact that the crucible would have a greater weight, when it later got burned off with the rest of the water, it would cause a higher perceived loss.
The fired crucible is handled with oily fingers before its mass measurement. Subsequently, the oil from the fingers is burned off. Will the percent water in the hydrated salt be reported as too high, too low or unaffected? If the oil from someone’s fingers were on the crucible it would cause an increase in the reported percent of water. This is because the initial measurement of the crucible to bee too high, then once the oil is burned off, it will be attributed to the experiment.
The crucible is handled with oily fingers after its mass measurement but before the 3 grams of hydrated salt is measured. Subsequently the oil from the fingers is burned off. Will the percent water in the hydrated salt be reported as too high, too low, or unaffected? If the crucible is contaminated after the initial measurement, but before the sample, the percentage of water loss will still be too high, causing a larger gap between the initial measurement of hydrated salt and final measurement of anhydrous salt.
Suppose the original sample is unknowingly contaminated with a second anhydrous salt. Will the percent water in the hydrated salt be recorded as too high, too low, or unaffected? If the sample was contaminated by another anhydrous salt after the firing, than the recorded water loss will be too low because the other salt will add mass, lessening the gap between the initial and final measurements.
The crucible is set on a lab bench, where it is contaminated with the cleaning oil used to clean the lab bench, but before its mass is measured. The analysis continues where the mass of the anhydrous salt is determine. While heating, the cleaning oil is burned off the bottom of the crucible. Describe the error that has occurred. If the crucible is set on the lab bench where it is contaminated by the cleaning oil before it is initially massed, it will have a greater perceived mass. However, once the oil is burned off it will cause the sample to appear that it has lost more of the sample than it truly had: meaning that the reported mass of the anhydrous salt will be too low.
The hydrated salt is overheated and the anhydrous salt thermally decomposes, one product being a gas. Will the reported percent water in the hydrated salt be reported too high, too low, or unaffected? If the sample is overheated causing the sample to thermally decompose, causing the product to be lost as gas, than the reported percentage of water loss will still be too high because the gap between the initial measurement and final (which will be lower due to loss of sample) measurement will widen.
Because of a lack of time, Bill decided to skip a step in the experimental procedure. Will his haste in reporting the percent H20 in the hydrated salt likely to be too high, too low or unaffected? If Bill decided to skip the experimental procedure, than we will not know if his results are too high or too low because we do not know what kind of error he made.
Some of the salt splattered because of overheating. Will the reported percent in hydrated water be too high, too low or unaffected? If the sample is overheated and some of the salt is lost due to it splattering out of the crucible, than the reported percentage of water loss will still be too high because the gap between the initial measurement and final (which is lower due to sample leaving the crucible) measurement will widen.

You Might Also Like