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Charra [1.4K]
3 years ago
8

The normal freezing point of a certain liquidXis0.4°C, but when5.90gof ureaNH22COare dissolved in450.gofX, it is found that the

solution freezes at−0.5°Cinstead. Use this information to calculate the molal freezing point depression constantKfofX.
Chemistry
1 answer:
mafiozo [28]3 years ago
3 0

Answer:

The molal freezing point depression constant for X is 4.12 °C/m

Explanation:

Apply the colligative property of freezing point depression:

ΔT = Kf . m . i

We assume the X lquid is non electrolytic, so i = 1

ΔT = T° freezing point of solvent pure - T° freezing point of solution

m = molality (moles of solute in 1kg of solvent)

Kf = Cryoscopic constant (the data we were asked for)

ΔT = 0.4° - (- 0.5°) = 0.9°

Let's calculate molality

Molar mass of urea = 60.06 g/m

Moles of urea = mass of urea / molar mass

5.90 g / 60.06 g/m = 0.098 moles

This moles are in 450 g of solvent, prepare a rule of three to find out the moles in 1000 g

450 g ____ 0.098 m

1000 g ____ ( 1000.  0.098)/450 = 0.218 m

0.9° = Kf . 0.218 m . 1

0.9° / 0.218 m = Kf → 4.12 °C/m

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Which of the following has a mass of 10.0 g?
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To solve this problem, we must understand that;

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Methanol, ethanol, and n−propanol are three common alcohols. When 1.00 g of each of these alcohols is burned in air, heat is lib
KengaRu [80]

Answer:

<u>For methanol:</u> Heat of combustion = -22.6 kJ / 0.0312 moles = -724.3590 kJ/mol (negative sign signifies release of heat)

<u>For ethanol: </u>Heat of combustion = -29.7 kJ / 0.0217 moles = -1368.6636 kJ/mol (negative sign signifies release of heat)

<u>For propanol: </u>Heat of combustion = -33.4 kJ / 0.0166 moles = -2012.0482 kJ/mol (negative sign signifies release of heat)

Explanation:

Given:

Mass of Methanol = 1.0 g

Mass of ethanol = 1.00 g

Mass of n-propanol = 1.00 g

<u>For methanol:</u>

2 CH₃OH + 3 O₂ ----> 2 CO₂ + 4 H₂O, ∆H₀ = -22.6 kJ/g  (negative sign signifies release of heat)

1 g of methanol on combustion gives 22.6 kJ of energy

Calculation of moles of methanol:

moles=\frac{Mass(m)}{Molar\ mass (M)}

Molar mass of methanol = 32.04 g/mol

Thus moles of methanol = 1 g/ (32.04 g/mol) = 0.0312 moles

Hence energy in kJ/mol:

<u>Heat of combustion = -22.6 kJ / 0.0312 moles = -724.3590 kJ/mol (negative sign signifies release of heat)</u>

<u></u>

<u>For ethanol:</u>

C₂H₅OH + 3 O₂ ----> 2 CO₂ + 3 H₂O, ∆H₀ = -29.7 kJ/g  (negative sign signifies release of heat)

1 g of ethanol on combustion gives 29.7 kJ of energy

Calculation of moles of ethanol:

moles=\frac{Mass(m)}{Molar\ mass (M)}

Molar mass of ethanol = 46.07 g/mol

Thus moles of ethanol = 1 g/ (46.07 g/mol) = 0.0217 moles

Hence energy in kJ/mol:

<u>Heat of combustion = -29.7 kJ / 0.0217 moles = -1368.6636 kJ/mol (negative sign signifies release of heat)</u>

<u></u>

<u>For propanol:</u>

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moles=\frac{Mass(m)}{Molar\ mass (M)}

Molar mass of methanol = 60.09 g/mol

Thus moles of methanol = 1 g/ (60.09 g/mol) = 0.0166 moles

Hence energy in kJ/mol:

<u>Heat of combustion = -33.4 kJ / 0.0166 moles = -2012.0482 kJ/mol (negative sign signifies release of heat)</u>

5 0
3 years ago
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