Determining the molar mass

A laboratory experiment on colligative properties directs students to determine the molar mass of an unknown solid. Each student receives 1.00 g of solute, 225 mL of solvent and information that may be pertinent to the unknown. 

The Rast method uses camphor (Ci0Hi O) as a solvent for determining the molar mass of a compound. When 2.50 g of cortisone acetate is dissolved in 50.00 g of camphor (kf = 40.0°C/m), the freezing point of the mixture is determined to be 173.44°C that of pure camphor is 178.40°C. What is the molar mass of cortisone acetate ... 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

We see that, for a given pressure and temperature, the greater the molar mass of the gas, the greater its density. Equation 10 also shows that, at constant temperature, the density of a gas increases with pressure. When a gas is compressed, its density increases because the same number of molecules are confined in a smaller volume. Similarly, heating a gas that is free to expand at constant pressure increases the volume occupied by the gas and therefore reduces its density. The effect of temperature on density is the principle behind hot-air balloons the hot air inside the envelope of the balloon has a lower density than that of the surrounding cool air. Equation 10 is also the basis for using density measurements to determine the molar mass of a gas or vapor. 

The van t Ho ff equation is used to determine the molar mass of a solute from osmotic pressure measurements. This technique, which is called osmometry, involves the determination of the osmotic pressure of a solution prepared by making up a known volume of solution of a known mass of solute with an unknown molar mass. Osmometry is very sensitive, even at low concentrations, and is commonly used to determine very large molar masses, such as those of polymers. 

Step 3 Determine the molar mass of the solute by dividing the given mass of solute, wtso utc (in grams) by the amount in moles (step 2). 

Osmosis is the flow of solvent through a semipermeable membrane into a solution the osmotic pressure is proportional to the molar concentration of the solute. Osmometry is used to determine the molar masses of compounds with large molecules, such as polymers reverse osmosis is used in water purification. 

Organic chemists once used freezing-point and boiling-point measurements to determine the molar masses of compounds that they had synthesized. When 0.30 g of a nonvolatile solute is dissolved in 30.0 g of CC14, the boiling point of the solution is 77.19°C. What is the molar mass of the compound ... 

Sections address how chemists answer the question, What is it It is just as important to ask, How much is there As described in Section 2-, chemists use the mole to describe amounts of substances. The procedures introduced in that section work equally well for compounds as for elements. To apply these procedures, we need to be able to determine the molar masses of chemical compounds. 

The line drawing and chemical formula of Sevin, a common insecticide, appear in the margin. Determine the molar mass of Sevin. 

To determine the molar mass of a substance, we need its chemical formula and elemental molar masses. From the chemical formula, determine the number of moles of each element contained in one mole of the substance. Multiply each elemental molar mass by the number of moles of that element, and add. 

Metal salts often exist as hydrates. One example is iron(II) nitrate hexahydrate. Determine the molar mass of this compound. 

Use the gas data to determine the molar mass. The problem gives the following data about the unknown... 

The problem has two parts. First we must calculate the density of atmospheric air. To do this, we need to determine the molar mass of diy air, which is the weighted average of the molar masses of its... 

C05-0072. Gaseous hydrocarbons, which contain only carbon and hydrogen, are good fuels because they bum in air to generate large amounts of heat. A sample of hydrocarbon with m— 1.65 g exerts a pressure of 1.50 atm in a 945-mL bulb at 21.5 °C. Determine the molar mass and chemical formula of this hydrocarbon. 

For determination of the molar mass of an unknown compound, a measured mass of material is dissolved to give a measured volume of solution. The system is held at constant temperature, and the osmotic pressure is determined. Osmotic pressure measurements are particularly useful for determining the molar masses of large molecules such as pol3TTiers and biological materials, as Example illustrates. 

C12-0018. When 7.50 mg of a protein is dissolved in water to give 10.00 mL of solution, the osmotic pressure is found to be 1.66 torr at 21°C. Determine the molar mass of the protein. 

Bravo, 1984). Hybrids of these systems, where chromatography and electrophoresis are used in each spatial dimension, were reported nearly 40 years ago (Efron, 1959). Belenkii and coworkers reported on the analysis of block copolymers by TLC (Gankina et al., 1991 Litvinova et al., 1991). Two-block copolymers of styrene and f-butyl methacrylate were separated first with regard to chemical composition by TLC at critical conditions, followed by a SEC-type separation to determine the molar masses of the components. 

In their first step towards building dendritic rotaxanes Stoddart et al. [140] reported the construction of such materials via a so-called slipping method (Fig. 24). Thus, treatment of bisparaphenylene-34-crown-10 (BPP34C10) with tris(bipyridinium) compound 54 at 50 °C in acetonitrile for 10 days afforded mono-, di-, and tris-rotaxanes 55, 56, and 57, respectively. ES-MS was used to determine the molar masses of these macromolecules while upfield shifts in the... 

In each case, we first determine the molar mass of the compound, and then the mass of the indicated element in one mole of the compound. Finally, we determine the percent by mass of the indicated element to four significant figures. 

A The balanced chemical equation provides the factor to convert from amount of Mg to amount of Mg3N2. First, we determine the molar mass of Mg3N2. 

We begin by determining the molar mass of Na2S04 10H2O. The amount of solute needed is computed from the concentration and volume of the solution. 

Determine the molar mass of each of the following to two decimal places, a. H20 b. C02 c. HN03 d. Na2S04 e. (NH4)3P04... 

This will give us the moles of calcium hydroxide instead of the grams, thus we need to add another step. We have moles and we need grams, the molar mass relates these two quantities. We determine the molar mass by using the formula and the atomic weights found in a table such as the periodic table. 

If it was not clear before, it should be clear now, that we still must find moles. We will find moles from the mass of KC103 and the balanced chemical equation. We need to determine the molar mass of KC103 from the atomic weights of the individual elements (122.55 g/mol). We now add our mole information to the equation ... 

A solution prepared by dissolving 7.95 mg of a gene fragment in 25.0 mL of water has an osmotic pressure of0.295 torr at 25.0°C. Assuming the fragment is a nonelectrolyte determine the molar mass of the gene fragment. 

In this example, we need to determine the molar mass (g/mol) of the gene fragment. This requires two pieces of information—the mass of the substance and the number of moles. We know the mass (7.95 mg), thus we need to determine the number of moles present. We will rearrange the osmotic pressure relationship to n 77 V/RT. We know the solute is a nonelectrolyte so i = 1. We can now enter the given values into the rearranged equation and perform a pressure and a volume conversion ... 

The osmotic pressure is a colligative property and mathematically can be represented as 71 = (nRTIV) i, where It is the osmotic pressure in atmospheres n is the number of moles of solute R is the ideal gas constant 0.0821 L atm/Kmol T is the Kelvin temperature Vis the volume of the solution and i is the van t Hoff factor. Measurements of the osmotic pressure can be used to calculate the molar mass of a solute. This is especially useful in determining the molar mass of large molecules such as proteins. 

When using the freezing-point depression method of determining the molar mass of a nonelectrolyte, what information is needed in addition to the above ... 

D—To calculate the molar mass, the mass of the solute and the moles of the solute are needed. The molality of the solution may be determined from the freezing-point depression, and the freezing-point depression constant (I and II). If the mass of the solvent is known, the moles of the solute may be calculated from the molality. These moles, along with the mass of the solute, can be used to determine the molar mass. 

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