molar mass of an unknown solid sample questions

Which statement best describes a colligative property?
a) Depends on chemical identity of the solute
b) Depends only on number of solute particles
c) Depends on solute mass and color
d) Depends on reactivity of solvent

B

Freezing-point depression occurs because:
a) Solute ions form ice crystals
b) Solute particles block solvent from forming a solid lattice
c) Solute increases vapor pressure
d) Solvent reacts chemically with solute

B

The freezing-point depression equation is:
a) ΔTₑ = Kₑ × C
b) ΔTₚ = Kₚ × moles
c) ΔT_f = K_f × m
d) m = K_f × ΔT_f

C

If ΔT_f doubles, what must have happened (assuming constant solvent mass)?
a) Half the solute added
b) Twice the solute added
c) Solvent evaporated
d) Solute dissolved less completely

B

What does ΔT_f represent?
a) Boiling-point rise
b) Temperature difference between solvent and solution freezing points
c) Temperature of plateau during heating
d) Cooling-curve slope

B

For cyclohexane, K_f = _____ °C·kg·mol⁻¹.
a) 1.86 

b) 3.9 

c) 20.0 

d) 8.0

C

T_f (pure cyclohexane) = _____ °C.
a) 0 

b) 20.0 

c) 8.0 

d) –1.86

C

Which of the following lowers freezing point the most?
a) 1 mol C₆H₁₂O₆ 

b) 1 mol NaCl 

c) 1 mol CH₃OH 

d) All equal

B

Molality (m) is defined as:
a) moles solute / liters solvent
b) moles solute / kg solvent
c) mass solute / kg solvent
d) moles solute / moles solvent

B

Which of the following has units of °C·kg/mol?
a) K_b 

b) K_f 

c) Molar mass 

d) Temperature

B

If ΔT_f = 1.8 °C and K_f = 20.0 °C·kg·mol⁻¹, what is molality (m)?
a) 0.09 m 

b) 0.36 m 

c) 0.009 m 

d) 18 m

A

Solvent mass = 13.3 g, m = 0.09 mol/kg.
Moles solute = ?
a) 0.00120 

b) 0.0120 

c) 0.00012 

d) 0.009

A

If 0.254 g solute = 0.00120 mol, what’s molar mass?
a) 212 g/mol 

b) 254 g/mol 

c) 103 g/mol 

d) 21 g/mol

A

When ΔT_f = 3.5 °C, K_f = 20.0, what’s m?
a) 0.175 

b) 0.35 

c) 1.75 

d) 2.0

A

A student used 0.497 g solute, 0.0133 kg solvent, ΔT_f = 3.5 °C, K_f = 20.0.
Approximate molar mass =?
a) 104 

b) 50 

c) 208 

d) 25

A

Which unit correctly expresses molality?
a) mol/L 

b) g/mol 

c) mol/kg 

d) mol/g

C

To find T_f (solution):
a) T_f (pure) + ΔT_f 

b) T_f (pure) – ΔT_f 

c) ΔT_f / K_f 

d) T_f (pure) × ΔT_f

B

ΔT_f = T_f (pure) – T_f (solution). If T_f (pure) = 8.0 °C, T_f (solution) = 4.5 °C, ΔT_f =?
a) 12.5 °C 

b) 3.5 °C 

c) 4.5 °C 

d) 8.0 °C

B

Why is the same solvent sample used for both trials?
a) To keep solute concentration constant
b) To ensure constant solvent mass for accuracy
c) To cool faster
d) To change K_f

B

Why stir during freezing?
a) Maintain even temperature and crystal formation
b) Speed up melting
c) Prevent evaporation
d) Keep thermometer from freezing

A

Why record temperature every 20–30 s?
a) Track the cooling curve precisely
b) Measure solubility
c) Avoid plateau formation
d) Reduce error in K_f

A

What is the plateau region on a cooling curve?
a) Temperature where solute dissolves
b) Region where solution and solid coexist; freezing point
c) Point before freezing starts
d) Lowest recorded temperature

B

Why use a cold-water or ice-salt bath?
a) To superheat solution
b) To freeze quickly and evenly
c) To evaporate solvent
d) To measure density

B

What happens if you overheat cyclohexane?
a) No effect
b) Increases ΔT_f
c) Solvent loss by evaporation → molar mass too low
d) Solvent gains mass

C

Why must the solute dissolve completely before cooling?
a) Undissolved solute gives smaller ΔT_f → molar mass too high
b) Prevents crystallization
c) Changes solvent density
d) No reason

A

When reading the plateau, you identify T_f where:
a) Curve first flattens for several readings
b) Lowest point occurs
c) Temperature rapidly drops
d) Solvent boils

A

What is the dependent variable in the experiment?
a) Mass solute 

b) ΔT_f 

c) K_f 

d) Solvent type

B

What is the independent variable?
a) Mass solute 

b) ΔT_f 

c) T_f (pure) 

d) K_f

A

Purpose of second trial with more solute:
a) Check proportionality of ΔT_f and solute amount
b) Verify solvent purity
c) Increase K_f value
d) Lower accuracy intentionally

A

Solute not fully dissolved →
a) ΔT_f too high → M too low
b) ΔT_f too low → M too high
c) ΔT_f same → M same
d) ΔT_f too low → M too low

B

Cyclohexane evaporated →
a) Less solvent → ΔT_f too high → M too low
b) More solvent → ΔT_f too low → M too high
c) No change
d) ΔT_f constant

A

Student added more solid than recorded mass →
a) M too high 

b) M too low 

c) K_f changes 

d) ΔT_f decreases

B

Cooling too fast →
a) Supercooling → harder to locate true plateau
b) ΔT_f increases
c) K_f decreases
d) None

A

Why does solvent evaporation make molar mass appear smaller?
a) ΔT_f smaller
b) ΔT_f larger (appears more concentrated)
c) Moles decrease
d) K_f increases

B

Error that increases apparent molar mass:
a) Evaporation
b) Undissolved solute
c) Spilled solid recorded as added
d) Too-fast cooling

B

If solute ionized (electrolyte), ΔT_f:
a) Decreases 

b) Increases 

c) No change 

d) K_f changes

B

Ionizing solute would make measured molar mass:
a) Too high 

b) Too low 

c) Unaffected 

d) Double

B

Large scatter between trials means:
a) Poor precision 

b) Good accuracy 

c) Systematic error 

d) No error

A

If ΔT_f₁ = 1.8 °C and ΔT_f₂ = 3.5 °C for double solute, results show:
a) ΔT_f ∝ solute mass (consistent)
b) Random error
c) Different solvent mass used
d) Solute not molecular

A

Why does solute lower freezing point?
a) It raises vapor pressure
b) It interferes with crystal lattice formation
c) It removes heat faster
d) It releases energy

B

What property remains constant for the same solvent?
a) K_f 

b) ΔT_f 

c) T_f (solution) 

d) Molality

A

Which property depends on solute identity?
a) K_f 

b) Color change 

c) ΔT_f 

d) Molality

B

If a solute dissociates into two particles, effective molality = ?
a) Half actual 

b) Equal 

c) Twice actual 

d) Zero

C

Why is freezing-point depression called a colligative property?
a) Depends on solvent polarity
b) Depends on number of solute particles
c) Depends on solute molar mass
d) Depends on chemical structure

B

What type of graph is produced in this lab?
a) Pressure vs Volume
b) Temperature vs Time (cooling curve)
c) Mass vs Temperature
d) Volume vs Temperature

B

The flat portion of the curve represents:
a) Cooling rate region
b) Phase equilibrium (liquid ⇌ solid)
c) Supercooling region
d) Evaporation

B

The equation m = ΔT_f / K_f is valid only if:
a) Solute is volatile
b) Solute is non-electrolyte and dilute
c) Solvent is ionic
d) Temperature is constant

B

If solute were ionic, what correction would be required?
a) Add ΔT_f 

b) Use van’t Hoff factor (i) in ΔT_f = i K_f m
c) Change solvent 

d) Ignore difference

B

What safety concern is unique to this lab?
a) Toxic vapors
b) Cyclohexane flammability
c) Glass under pressure
d) Acid burns

B

Proper disposal of solutions:
a) Sink drain
b) Acidic aqueous waste
c) Organic liquid waste
d) Solid bin

C