To calculate how much the freezing point of the solvent is depressed or the boil

Question

To calculate how much the freezing point of the solvent is depressed or the boiling point of the solvent is elevated, we use the following equations: ATf Kf. msolute Freezing point depression ATb-Kb.m Boiling point elevation The Kf and Kb values are based on the solvent, not the solute, and are in units of°C per molal. Normal boiling point Solvent Normal freezing point Water 100.0 0.515 0.0 1.86 Ethanol 784 1.22 -114.6 1.99 Benzene so. 2. SS 5.12 76.8 tetrachloride chloroform 61.2 3.62 -63.5 4.68 Practice Problems: l. How many grams of NaCl would be required to raise the boiling temperature of 100 g of water by 3.00 oC? ATf 3.00 °C (0.51 °C/m00mparticles) mpanicles 5.9 mol particles/kg water 5.9 mol particles (0.100 kg) 0.59 mol partcles 1 kg water Each NaCl unit makes 2 particles, so while we have 0.59 moles of particles, we only need half that of NaCl, or 0.29 mol NaCl. (0.29 mol NaCl (58.44 g/mol) 17 g Naci.

Explanation / Answer

1)
here mb is molality. Find molality first
i is number of particle a solute give. Here it is 2
delta Tb = i*Kb*mb
3 = 2*0.515*mb
mb = 2.91 mol/Kg

now find mol of NaCl using,
mb = number of mol / mass of water in Kg
2.91 mol/Kg= number of mol/0.1 Kg
number of mol = 0.29 mol

This is number of mol of NaCl

we know that molar mass of NaCl = 58.44 g/mol

mass of NaCl = number of mol * molar mass
= 0.29 mol * 58.44 g/mol
=17 g

Answer: 17 g

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