Chemistry Forum

i've been working on these probs for a while and i can't seem to get them...my head hurts, any help would be wonderful. thanks!!

1. you are asked to design an airbag for a car. the bag should be filled w/ gas w/ a pressure higher than atmospheric pressure, say 828mmHg, at a temp of 22.0 C. the bag has volume of 45.5L. what quantity of sodium azide (Nsub₃sup-1), should be used to generate the required quantity of gas? the gas producing rxn is sodium azide decomposes into sodium solid and nitrogen gas.

2. calculate change in enthalpy for the reaction: 4A + 2B + 5D + 2E + H + 2G --> 4F + C

2E + 6F + 2G --> 4A + 6B + 2C + 4D (Hr = 24kJ)
1/2G + B --> 2D + A + 3/2E + 1/2F (Hr = -32 kJ)
6F + 9C + 3D --> 12G + 3C + 3B + 3H (Hr = 47 kJ)

3. imagine you dissolved 2.00g of Csub₆Hsbu₅COsub₂H and 2.00g of NaCsub₆Hsub₅COsub₂ in enough water to make 1.00L of solution. if pH of solution is 4.13, determine pKa.
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for 1, i don't see the connection of sodium azide w/ anything, i thought the gas you're using to fill it is Hg??

for 2, i just CAN'T get it. i have no idea what to reverse or double...even just a hint would be great. i just need to freakin' solve it!

for 3, do i make 2 reactions or 1 (where i dissolve both acids together in the water)?? confuzzled.

Firstly, mmHg is a measurement of pressure. Treat it like you would meters, Celsius or liters.

1) This question isn't worded properly. I think it means that you need to know the quantity of sodium azide it would take to achieve a pressure of 828 mmHg. Let's assume the pressure is going to be atmospheric to start with.

The reaction is : You know that PV = nRT. Since PV / RT = n, we can find the number of moles of nitrogen that we need to fill up the bag.

Let's convert mmHg to atm. (828/760) = 1.09 atm.

Now let's convert Celcius to Kelvin (because I don't know the ideal gas constant at anything other than K atm and L. ).

( 45.5 L * 0.09 atm <(notice we're using 0.09 because the air is going to be at 1.00 anyway, not 0.0. ) / (295.15 K * 0.0821) = 0.169 moles of gas.

Now we know how many moles of azide we need. Since, according to the equation, every two moles of sodium azide makes three moles of gas, we need (2/3) times as many moles of sodium azide as gas. This means we need 0.123 moles of sodium azide.

The molecular weight of sodium azide is 65.01 g / mol. Multiplying these, we get that we need 8.00 grams of sodium azide to achieve the required pressure.




Moving right along :

2) What a nightmare! I'm gonna name the equations a), b) and c) from top to bottom. Notice equation c) is the only one with an 'H' in it. Since the solution has one 'H' as a reactant, the coefficient for this one must be -(1/3). Now that we know the coefficient for c), we can note that equation b) doesn't have any 'C's in it! This means that to balance for the solution, we must manipulate a) such that the 'C's are balanced in the solution.

Currently, there are four 'C's on the right side when there should be one. We have to reverse the top equation, then, and halve it. The coefficient for a) is -(1/2).

The last one is simple. 'A's must now be balanced. Since there are Now 3 A's on the right and we need four, let's simply multiply the middle by -2.

And that's the answer.

a) -(1/2)
b) -2
c) -(1/3)

36.3 kJ.


3) No equations needed. You're making a buffer solution.

Use the Henderson/Hasselbach equation.

pH - log ([base]/[acid]) = pKa

Convert to molarity.
2.00 g / 122.12 g /mol = 0.0164 M benzoic acid.
2.00 g / 144.11 g / mol = 0.0139 M sodium benzoate.
pH = 4.13.

4.13 - log ([0.0139]/[0.0164]) = pKa

pKa = 4.20.

This is good, it is close to the reported value of benzoic acid's pKa, which is 4.21.