Lab Exam 😰 Flashcards
Mass of water in a hydrated sample=
Original mass of sample - mass of sample after heating
Mass % water in a hydrate:
100(mass water in the sample/original mass of the sample)
Changes a hydrate may undergo as it is heated
1) colour change
2) change in crystal form which usually results in a dry powdery solid
3) it may dissolve in its own water of crystallization/hydration
Anhydrous form
When all the water has been driven out of a hydrated compound
Dessicant
Another term for hygroscopic compound-compound that spontaneously absorbs water from the atmosphere
-widely used in the industrial drying of liquids/gases
Delinquescent
A hygroscopic compound (absorbs water spontaneously) that absorbs so strongly, it dissolves in its own water of hydration
Hygroscopic compound
Compound that spontaneously absorbs water from the atmosphere
Efflorescent
Hydrates that spontaneously lose water to the atmosphere
Chemical formula of sucrose
C12H22O11
Upon hearing an unknown hydrate, no visible change occurs. How would you decide if water had been completely driven off?
Weigh the heated sample and compare it to the initial mass. If the heated sample weighs less, then water has been driven off. Reheat and reweigh; if the mass REMAINS CONSTANT then all of the water has been driven off.
How could you tell by observing samples weather a compound has a greater vapour pressure than the partial vapour pressure of water in the atmosphere?
If it has a greater vapour pressure (ex. CdCl2), it would efflorescence (lose water spontaneously) while a substance with a lower vapour pressure (ex. CaCl2) would gain water spontaneously.
Why doesn’t sucrose reform on adding water to its after-heating residue?
Combustion is an irreversible chemical change whereas driving off the water in a hydrate is a physical change.
Vitamin C
Also called ascorbic acid
Water-soluble due to the formation of hydrogen bonds
When titrating vitamin C and tang with iodate and NaOH, what is the difference in the reactions?
Iodate (IO3-) reacts only with ascorbate, the acidic proton resulting from the Vitamin C, while NaOH reacts with the acid protons from any (and all) acids present.
Explain the difference between endpoint and equivalence point
Endpoint refers to the part of a reaction during which both substances attain an exact stoichiometric molar ratio
Equivalence point is a visual indication (usually a colour change from an indicator) that signals the reaction’s completion
