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Purifying water

Submitted by sugerkissess on Sun, 03/09/2008 - 18:53

Having trouble solving these 3 problems. please help or advise.

1. When camping in remote locations, water can be in short supply. It is not
practical to carry all of the water needed during a trip from the starting
point to the campsite. Hikers often look for other sources of potable
(drinkable) water near their campsite. During the winter months, it is
possible to melt clean snow and turn it into drinkable water. To warm
themselves, campers make pots of “trail coffee” – coffee grounds boiled in
a pot with water.

a. Two hikers in Acadia National Park melt snow to make 1000. mL of
trail coffee. If the snow used is originally at –10 °C, how much
energy is required to heat the water to its boiling point? [Cp(ice) =
37.1 J/mol °C; ?Hfus = 6.01 kJ/mol; Cp(H2O) = 75.3 J/mol °C]
Assume that the density of snow and liquid water are both 1.00
g/mL.

b. To boil the water, a small propane-fueled stove is commonly used.
Given that the heat of combustion (?H°c) of propane (C3H8) is –
2220. kJ mol-1, please calculate the number of moles of propane
needed to melt the water.

c. Propane is usually sold in 400 g fuel canisters. How many canisters
would you need to make the coffee?

2. A 1.00% by mass NaCl(aq) solution has a freezing point of -0.192 C.

a. Estimate the van’t Hoff factor (i) from the data.

b. Determine the molal concentrations of Na+. Cl-, and NaCl.

c. Calculate the percent dissociation of NaCl in this solution.
(Hint: When calculating freezing point depression, the
concentration (m*i) is the sum of the molalities of Na+. Cl-, and
undissociated NaCl.)

3. In the human liver, the oxidation of ethanol (from alcoholic beverages) to
acetaldehyde, catalyzed by the enzyme by alcohol dehydrogenase, is a
zeroth-order reaction.
CH3CH2OH + NAD+ ? CH3CHO + NADH + H+
An average 70-kg person typically takes about 2.5 hrs to oxidize the 15 mL
of ethanol (C2H5OH; density = 0.7893 g mL-1) contained in a single 12-oz
can of beer, 5-oz glass of wine, or one shot of distilled spirits.

a. Which plot will be linear - [ethanol] vs. time, ln [ethanol] vs. time,
or 1/[ethanol] vs. time?

b. What is the rate law for this reaction?

c. What is the value of the rate constant for this reaction? Be sure to
include the proper units.

d. The actual rate of oxidation varies greatly from person to person.
What causes this discrepancy? (Assume that all people have the
same body temperature of 98.6 ºF.)


a. Two hikers in Acadia National Park melt snow to make 1000. mL of
trail coffee. If the snow used is originally at –10 °C, how much
energy is required to heat the water to its boiling point? [Cp(ice) =
37.1 J/mol °C; ?Hfus = 6.01 kJ/mol; Cp(H2O) = 75.3 J/mol °C]
Assume that the density of snow and liquid water are both 1.00
g/mL.

To do this we need to measure the amount of heat needed to do three things to the water:

1)Heat the snow to the melting point (from -10C to 0C)
2)Melt the snow
3)Heat the melted snow from 0C to 100C.

Steps 1 and 3 involve a temperature change.  Any time a heat problem involves a temperature change, the following equation is used:
                         heat = moles of sample  x molar heat capacity of sample x change in temp of sample.

You will need to convert the mass of snow/water into moles.  You are given the molar specific heat capacities of snow and water (Cp) (Note they will be different for the two steps), and you can determine the change in temperature for each step.

In step 2 there is a phase change.  The Heat of fusion indicates the amount of energy required to melt 1 mole of snow.  Multiplying the heat of fusion by the moles of snow will yield the amount of energy requirred to melt the snow.

Add the energy need for the three steps together to determine the total amount of energy required for the task

In b the stove will need to provide the amount of heat that you calculated above,  this quantity of heat, along with the heat of combustion of propane provided in the question can be used to calculate the amount of propane required for the task.