[lrw1793]

Hi, I'd just like to check my method for working this out isn't wrong, the last answer seems too large a value to me! Sorry if it's hard to follow

Air is composed of 80% nitrogen at sea level where the pressure is 1 bar.
A human with a body weight of 80 kg has an average blood volume of 5.6 L.
Henry’s law constant for the solubility of nitrogen in water: 9.04 × 104 bar at 298 K.
Assume that blood has identical properties to water (Henry’s law constant, density and relative molecular mass).

At sea level, calculate the number of moles of nitrogen absorbed in the blood of a human with a body weight of 80 kg.
5.6L of blood RMM = 20
5.6L = 5.6 dm³
xsolute = Psolute/kHsolute
1/9.04x10⁴ = 1.106x10^-5
xsolute = 1.106x10-5
nsolute = 1.106x10^-5 x nsolvent
mass of water = volume x density = 5600cm³ x 1g cm^-3 = 5600g
nsolvent = mass/RMM = 5600/18 = 311.1 moles
nsolute = 1.106x10^-5 x 311.1 = 3.44x10^-3 moles

At a pressure of 5 bar, calculate the number of moles of nitrogen absorbed in the blood of a human with a body weight of 80 kg.
P/kH = xsolute
5/9.04x10⁴ = 5.531x10^-5
nsolute = xsolute x nsolvent = 5.531x10^-5  x 311.1 = 0.017 moles

Outline briefly how this equation is obtained from first principles.
I have no idea on this one, does it mean how it's derived?

Assume that a diver accustomed to breathing compressed air at a pressure of 5 bar is suddenly brought to sea level. What volume of N2 gas is released as bubbles in the diver’s bloodstream?
Gas constant: 8.314 × 10–2 L bar mol–1 K–1
At 5 bar diver has 0.017 moles N2 in blood
At 1 bar (sea level) 3.44 x10-3 moles
0.017 - 3.44 x10-3  = 0.01356 moles of N2
pV = nRT
V = nRT/P
V = (0.01356 x 8.314x10-2 x 298)/1
V = 0.33 L - this just seems like too much to me if it's only 0.01356 moles nitrogen?

[Borek]

Why RMM of water is 20?

Pressure of nitrogen is not 1 atm.

You are omitting units everywhere, which makes following your calculations pretty hard, but they are already wrong here.

[lrw1793]

Why RMM of water is 20?

Pressure of nitrogen is not 1 atm.

You are omitting units everywhere, which makes following your calculations pretty hard, but they are already wrong here.

Oh I don't know why I wrote that, I did the actual calculation with 18. No, the question says it's 1 bar.

[lrw1793]

Why RMM of water is 20?

Pressure of nitrogen is not 1 atm.

You are omitting units everywhere, which makes following your calculations pretty hard, but they are already wrong here.

Added the missing units here:
5.6L of blood RMM = 18
5.6L = 5.6 dm³
xsolute = Psolute/kHsolute
1 bar/9.04x10⁴bar = 1.106x10^-5
xsolute = 1.106x10^-5
nsolute = 1.106x10^-5 x nsolvent
mass of water = volume x density = 5600cm³ x 1g cm^-3 = 5600g
nsolvent = mass/RMM = 5600g/18 = 311.1 moles
nsolute = 1.106x10^-5 x 311.1moles = 3.44x10^-3 moles

[Borek]

No, the question says it's 1 bar.

No, it doesn't say that.

[lrw1793]

No, the question says it's 1 bar.

No, it doesn't say that.

Oh I see, so I need to work out the pressure of N₂ dissolved in the blood using this Psolute = nsolute/nsolvent kHsolute?

5 bar: has 0.017 moles N₂ in blood
1 bar: 3.44 x10^-3 moles N₂ in blood
0.017 moles - 3.44 x10^-3 moles = 0.01356 moles of N₂

Psolute = nsolute/nsolvent kHsolute = (0.01356 moles/311.1 moles) x  9.04 × 104 bar = 3.94 bar


pV = nRT
V = nRT/p

V = (0.01356 moles x 8.314x10^-2 bar mol^–1 K^–1 x298K)/3.94 bar
V = 0.085 L

[Borek]

No, the question says it's 1 bar.

No, it doesn't say that.

Oh I see, so I need to work out the pressure of N₂ dissolved in the blood using this Psolute = nsolute/nsolvent kHsolute?

You are doing convoluted things, but you are still ignoring crucial part of the information about nitrogen, that is given in the problem.

Hint: 1 bar is a sum of partial pressures of all gases in the air.

[lrw1793]

No, the question says it's 1 bar.

No, it doesn't say that.

Oh I see, so I need to work out the pressure of N₂ dissolved in the blood using this Psolute = nsolute/nsolvent kHsolute?

You are doing convoluted things, but you are still ignoring crucial part of the information about nitrogen, that is given in the problem.

Hint: 1 bar is a sum of partial pressures of all gases in the air.

Ah, got it. 80% of 1 bar so 0.8 bar. Need to learn to read the question properly  thanks!