CHEM-120 UNIT 7 LAB PRACTICUM 2026
PRACTICE SET LAB REVIEW MOCK
ASSESSMENT KEY SOLUTIONS A PLUS
◉ equation relating freezing point and van't Hoff factor Answer: ∆t_f
= iK_fm
◉ In the freezing point depression experiment, what temperature is
the ice/salt bath supposed to be at? Answer: -10 ºC
◉ In the freezing point depression experiment, what was the solute?
Solvent? Answer: solute: isopropyl alcohol
solvent: water
◉ percent error equation Answer: | (experimental - theoretical)/
theoretical | x 100%
◉ difference b/t molarity and molality Answer: molarity: mols
solute/ L solution
molality: mols solute/ kg solvent
,◉ As van't Hoff factor increases, does freezing point increase or
decrease? Answer: decrease
◉ If the calculated molar mass of a compound is 10, but the actual
molar mass is 5, what should the van't Hoff factor be? Answer: 1/2
◉ What does the green dye react with that causes its color to
change? What color does it change to? Is this the result of a
structural change? Answer: - a Lewis base
- colorless
- yes
◉ aromatic compound Answer: contains plantar ring systems with
alternating double bonds; ex: dyes
◉ chromophore Answer: the part of the molecule responsible for its
color
◉ As more methlyl groups are added, what happens to the color of a
dye? Answer: it changes (from red to reddish purple to blue to
green)
◉ does methyl green have resonance structures? does it act as a
Lewis acid or a Lewis base? Answer: - yes
, - Lewis acid
◉ The color of methyl green fades under ____ (acidic/basic)
conditions. The reaction is ______(faster/slower) as conditions
continue in this fashion (as the dye fades). Answer: basic; faster
◉ net reaction involving Na2HPO4 and Na3PO4 buffer system
(experiment 2 - rate of decolorization) Answer: PO4^3- + H2O(l) -
><- HPO4^-2 + OH^-
◉ What order is the reaction involving Na2HPO4 and Na3PO4 buffer
system? (experiment 2 - rate of decolorization) Answer: first order
with respect to the dye
◉ what is the observed rate law for the reaction involving Na2HPO4
and Na3PO4 buffer system? (experiment 2 - rate of decolorization)
Answer: rate = k1 [dye][H2O] + k2[dye][OH-] or [dye]x(k_obs)
◉ When a reaction is first order, but [h2O]>>[dye] , what will
happen? What will the observed rate constant be in this case?
Answer: the reaction will be pseudo first order
k (obs) = k1[H2O] + k2[OH-]
PRACTICE SET LAB REVIEW MOCK
ASSESSMENT KEY SOLUTIONS A PLUS
◉ equation relating freezing point and van't Hoff factor Answer: ∆t_f
= iK_fm
◉ In the freezing point depression experiment, what temperature is
the ice/salt bath supposed to be at? Answer: -10 ºC
◉ In the freezing point depression experiment, what was the solute?
Solvent? Answer: solute: isopropyl alcohol
solvent: water
◉ percent error equation Answer: | (experimental - theoretical)/
theoretical | x 100%
◉ difference b/t molarity and molality Answer: molarity: mols
solute/ L solution
molality: mols solute/ kg solvent
,◉ As van't Hoff factor increases, does freezing point increase or
decrease? Answer: decrease
◉ If the calculated molar mass of a compound is 10, but the actual
molar mass is 5, what should the van't Hoff factor be? Answer: 1/2
◉ What does the green dye react with that causes its color to
change? What color does it change to? Is this the result of a
structural change? Answer: - a Lewis base
- colorless
- yes
◉ aromatic compound Answer: contains plantar ring systems with
alternating double bonds; ex: dyes
◉ chromophore Answer: the part of the molecule responsible for its
color
◉ As more methlyl groups are added, what happens to the color of a
dye? Answer: it changes (from red to reddish purple to blue to
green)
◉ does methyl green have resonance structures? does it act as a
Lewis acid or a Lewis base? Answer: - yes
, - Lewis acid
◉ The color of methyl green fades under ____ (acidic/basic)
conditions. The reaction is ______(faster/slower) as conditions
continue in this fashion (as the dye fades). Answer: basic; faster
◉ net reaction involving Na2HPO4 and Na3PO4 buffer system
(experiment 2 - rate of decolorization) Answer: PO4^3- + H2O(l) -
><- HPO4^-2 + OH^-
◉ What order is the reaction involving Na2HPO4 and Na3PO4 buffer
system? (experiment 2 - rate of decolorization) Answer: first order
with respect to the dye
◉ what is the observed rate law for the reaction involving Na2HPO4
and Na3PO4 buffer system? (experiment 2 - rate of decolorization)
Answer: rate = k1 [dye][H2O] + k2[dye][OH-] or [dye]x(k_obs)
◉ When a reaction is first order, but [h2O]>>[dye] , what will
happen? What will the observed rate constant be in this case?
Answer: the reaction will be pseudo first order
k (obs) = k1[H2O] + k2[OH-]
