<< Chapter < Page Chapter >> Page >
This figure shows three scenarios relating to red blood cell membranes. In a, H subscript 2 O has two arrows drawn from it pointing into a red disk. Beneath it in a circle are eleven similar disks with a bulging appearance, one of which appears to have burst with blue liquid erupting from it. In b, the image is similar except that rather than having two arrows pointing into the red disk, one points in and a second points out toward the H subscript 2 O. In the circle beneath, twelve of the red disks are present. In c, both arrows are drawn from a red shriveled disk toward the H subscript 2 O. In the circle below, twelve shriveled disks are shown.
Red blood cell membranes are water permeable and will (a) swell and possibly rupture in a hypotonic solution; (b) maintain normal volume and shape in an isotonic solution; and (c) shrivel and possibly die in a hypertonic solution. (credit a/b/c: modifications of work by “LadyofHats”/Wikimedia commons)

Determination of molar masses

Osmotic pressure and changes in freezing point, boiling point, and vapor pressure are directly proportional to the concentration of solute present. Consequently, we can use a measurement of one of these properties to determine the molar mass of the solute from the measurements.

Determination of a molar mass from a freezing point depression

A solution of 4.00 g of a nonelectrolyte dissolved in 55.0 g of benzene is found to freeze at 2.32 °C. What is the molar mass of this compound?

Solution

We can solve this problem using the following steps.

This is diagram with five boxes oriented horizontally and linked together with arrows numbered 1 to 4 pointing from each box in succession to the next one to the right. The first box is labeled, “Freezing point of solution.” Arrow 1 points from this box to a second box labeled, “delta T subscript f.” Arrow 2 points from this box to to a third box labeled “Molal concentration of compound.” Arrow labeled 3 points from this box to a fourth box labeled, “Moles of compound in sample.” Arrow 4 points to a fifth box labeled, “Molar mass of compound.”
  1. Determine the change in freezing point from the observed freezing point and the freezing point of pure benzene ( [link] ).
    Δ T f = 5.5 ° C 2.32 ° C = 3.2 ° C
  2. Determine the molal concentration from K f , the freezing point depression constant for benzene ( [link] ), and Δ T f .
    Δ T f = K f m m = Δ T f K f = 3.2 ° C 5.12 ° C m −1 = 0.63 m
  3. Determine the number of moles of compound in the solution from the molal concentration and the mass of solvent used to make the solution.
    Moles of solute = 0.62 mol solute 1.00 kg solvent × 0.0550 kg solvent = 0.035 mol
  4. Determine the molar mass from the mass of the solute and the number of moles in that mass.
    Molar mass = 4.00 g 0.034 mol = 1.2 × 10 2 g/mol

Check your learning

A solution of 35.7 g of a nonelectrolyte in 220.0 g of chloroform has a boiling point of 64.5 °C. What is the molar mass of this compound?

Answer:

1.8 × 10 2 g/mol

Got questions? Get instant answers now!

Determination of a molar mass from osmotic pressure

A 0.500 L sample of an aqueous solution containing 10.0 g of hemoglobin has an osmotic pressure of 5.9 torr at 22 °C. What is the molar mass of hemoglobin?

Solution

Here is one set of steps that can be used to solve the problem:

This is a diagram with four boxes oriented horizontally and linked together with arrows numbered 1 to 3 pointing from each box in succession to the next one to the right. The first box is labeled, “Osmotic pressure.” Arrow 1 points from this box to a second box labeled, “Molar concentration.” Arrow 2 points from this box to to a third box labeled, “Moles of hemoglobin in sample.” Arrow labeled 3 points from this box to a fourth box labeled, “Molar mass of hemoglobin.”
  1. Convert the osmotic pressure to atmospheres, then determine the molar concentration from the osmotic pressure.
    Π = 5.9 torr × 1 atm 760 torr = 7.8 × 10 −3 atm Π = MRT M = Π R T = 7.8 × 10 −3 atm ( 0.08206 L atm/mol K ) ( 295 K ) = 3.2 × 10 −4 M
  2. Determine the number of moles of hemoglobin in the solution from the concentration and the volume of the solution.
    moles of hemoglobin = 3.2 × 10 −4 mol 1 L solution × 0.500 L solution = 1.6 × 10 −4 mol
  3. Determine the molar mass from the mass of hemoglobin and the number of moles in that mass.
    molar mass = 10.0 g 1.6 × 10 −4 mol = 6.2 × 10 4 g/mol

Check your learning

What is the molar mass of a protein if a solution of 0.02 g of the protein in 25.0 mL of solution has an osmotic pressure of 0.56 torr at 25 °C?

Answer:

2.7 × 10 4 g/mol

Got questions? Get instant answers now!

Colligative properties of electrolytes

As noted previously in this module, the colligative properties of a solution depend only on the number, not on the kind, of solute species dissolved. For example, 1 mole of any nonelectrolyte dissolved in 1 kilogram of solvent produces the same lowering of the freezing point as does 1 mole of any other nonelectrolyte. However, 1 mole of sodium chloride (an electrolyte) forms 2 moles of ions when dissolved in solution. Each individual ion produces the same effect on the freezing point as a single molecule does.

Questions & Answers

A stone is dropped down a well, if it take 5 seconds to reach the water, how dip is the well
Mollamin Reply
an aircraft at as steady velocity of 70m/so eastwards at a height of 800me drops a package of supplies .a, how long will it take for the package to rich the ground? b, how fast will it be going as it lands?
Ng Reply
what is hypothesis theory law
Tamba
physics is the science of measurement
Jide Reply
What is physics
Victor Reply
what is physics
Obaro
Good question! Physics is the study of the nature world . Does this help?
Yonn
physics is the study of matter in relations to energy.
Enoch
physics is the science of measurements
Jide
physics is a science concern with nature and properties of matter and energy
Ugomma
what is a parallelogram law of motion?
Nancy
Definition for physics
Adesola Reply
It deal with matter and relation to energy
Soughie
physics is the Study of matter in relation to energy.
albert
physics is a natural science that study matter its behaviour and relation to energy.
mohammed
physic tells us more about quantities and measurement also
Kelly
life as we know it that can be measured and calculated
Jesus
what is a reference frame
Chukwu Reply
what is anatomy in relation to physics
Mubarak Reply
how does half life exist
Humble Reply
 The amount of time it takes a radioactive isotope to decay into a stable isotope is different for each radioactive isotope, and is characterized by its “half-life”. An isotope's half-life is the amount of time it takes for half the number of atoms of that isotope to decay to another isotope.
Nardine
what is the difference between Mass and weight
Pjustin
mass is constant while weight varies. unit of mass is kg, unit of weight is newton
Faith
how can a coin float in water and what principle governs it
Mercy Reply
in my opinion that work of surface tension but restrictions on coin is that coin do not break surface energy of molecules but some days before scientists prove that's another types of force
Aman
which force hold floating coins together thats my confusion
Aman
how can a coin float in water and what principle governs it
Mercy Reply
why many of the coin floating in water
Aman Reply
what is hook's law
Momoh Reply
provided the elastic limit is not exceeded,the extension produce in an elastic material is equal to the applied force
Mahmud
What is Andromeda
Chinecherem Reply
👡
Mahmud
What are the international agreed system of unit for physical measurements
Aisher Reply
m, kg, s
Faith
kg , m, s
Faith
si
Aman
meter , kilogram , second
Kelly
international system of units is agreed system because more units are not in mks system so si exist with ampere unit
Aman
what is thermodynasmics
Femkid Reply
what is electromagnetic force
Femkid
Electromagnetic force means a force that deals with electric and magnetic
Nebil
how do you find theWhat are the wavelengths and energies per photon of two lines
caroline Reply
The eyes of some reptiles are sensitive to 850 nm light. If the minimum energy to trigger the receptor at this wavelength is 3.15 x 10-14 J, what is the minimum number of 850 nm photons that must hit the receptor in order for it to be triggered?
razzyd Reply
A teaspoon of the carbohydrate sucrose contains 16 calories, what is the mass of one teaspoo of sucrose if the average number of calories for carbohydrate is 4.1 calories/g?
ifunanya Reply
4. On the basis of dipole moments and/or hydrogen bonding, explain in a qualitative way the differences in the boiling points of acetone (56.2 °C) and 1-propanol (97.4 °C), which have similar molar masses
Kyndall Reply
Calculate the bond order for an ion with this configuration: (?2s)2(??2s)2(?2px)2(?2py,?2pz)4(??2py,??2pz)3
Gabe Reply
Which of the following will increase the percent of HF that is converted to the fluoride ion in water? (a) addition of NaOH (b) addition of HCl (c) addition of NaF
Tarun Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Ut austin - principles of chemistry. OpenStax CNX. Mar 31, 2016 Download for free at http://legacy.cnx.org/content/col11830/1.13
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Ut austin - principles of chemistry' conversation and receive update notifications?

Ask