molarity

Examples of molarity in the following topics:

• Using Molarity in Calculations of Solutions

  • In chemistry, molar concentration, or molarity, is defined as moles of solute per total liters of solution.
  • The SI unit for molarity is is mol/m3; however, you will almost always encounter molarity with the units of mol/L.
  • This video demonstrates practice problems with molarity, calculating the moles and liters to find the molar concentration.
  • Use molarity to convert between mass and volume in a solution.
  • This video looks at how to use molarity as a conversion factor.

• Molar Mass of Gas

  • To find the molar mass of a mixture of gases, you need to take into account the molar mass of each gas in the mixture, as well as their relative proportion.
  • The average molar mass of a mixture of gases is equal to the sum of the mole fractions of each gas, multiplied by their respective molar masses:
  • where m is the mass of the gas, and M is the molar mass.
  • Finally, putting the equation in terms of molar mass, we have:
  • What is the molar mass of the gas?

• Mass-to-Mole Conversions

  • Mass-to-mole conversions can be facilitated by employing the molar mass as a conversion ratio.
  • The compound's molar mass is necessary when converting from grams to moles.
  • For a single element, the molar mass is equivalent to its atomic weight multiplied by the molar mass constant (1 g/mol).
  • For a compound, the molar mass is the sum of the atomic weights of each element in the compound multiplied by the molar mass constant.
  • The molar mass of water is 18 g/mol.

• Molar Ratios

  • Molar ratios, or conversion factors, identify the number of moles of each reactant needed to form a certain number of moles of each product.
  • From this reaction equation, it is possible to deduce the following molar ratios:
  • These molar ratios can also be expressed as fractions.
  • These molar ratios will be very important for quantitative chemistry calculations that will be discussed in later concepts.
  • Calculate the molar ratio between two substances given their balanced reaction

• Molar Mass of Compounds

  • The molar mass of a particular substance is the mass of one mole of that substance.
  • The characteristic molar mass of an element is simply the atomic mass in g/mol.
  • However, molar mass can also be calculated by multiplying the atomic mass in amu by the molar mass constant (1 g/mol).
  • The molar mass of NaCl is 58.44 g/mol.
  • This video shows how to calculate the molar mass for several compounds using their chemical formulas.

• Molarity

  • Molarity is defined as the moles of a solute per volume of total solution.
  • The SI unit for molar concentration is mol/m3.
  • However, mol/L is a more common unit for molarity.
  • To calculate the number of moles in a solution given the molarity, we multiply the molarity by total volume of the solution in liters.
  • What is the molarity of the new solution?

• Converting between Mass and Number of Moles

  • For example, if the atomic mass of sulfer (S) is 32.066 amu, then its molar mass is 32.066 g/mol.
  • In a compound of NaOH, the molar mass of Na alone is 23 g/mol, the molar mass of O is 16 g/mol, and H is 1 g/mol.
  • What is the molar mass of NaOH?
  • Since the molar mass of NaOH is 40 g/mol, we can divide the 90 g of NaOH by the molar mass (40 g/mol) to find the moles of NaOH.
  • What is the molar mass of methane (CH4) if there are 0.623 moles in a 10.0g sample?

• Molar Solubility and Relative Solubility

  • The units are molarity (M), or mole liter-1 (mol/L).
  • The relation between the molar solubility and the solubility product means that one can be used to find the other.
  • What is the molar solubility?
  • Compare their molar solubilities in water at 25 °C.
  • Calculate the molar solubility of a compound in water and the relative molar solubility of compounds

• Mass-to-Mass Conversions

  • Therefore, for a mass-to-mass conversion, it is necessary to first convert one amount to moles, then use the conversion factor to find moles of the other substance, and then convert the molar value of interest back to mass.
  • Taking coefficients from the reaction equation (13 O2 and 2 C4H10), the molar ratio of O2 to C4H10 is 13:2.
  • The molar amount of O2 can now be easily converted back to grams of oxygen:
  • But by converting the butane mass to moles (0.929 moles) and using the molar ratio (13 moles oxygen : 2 moles butane), one can find the molar amount of oxygen (6.05 moles) that reacts with 54.0 grams of butane.
  • Using the molar amount of oxygen, it is then possible to find the mass of the oxygen (193 g).

• Molality

  • Unlike molarity, which depends on the volume of the solution, molality depends only on the mass of the solvent.
  • Since volume is subject to variation due to temperature and pressure, molarity also varies by temperature and pressure.
  • Compared to molar concentration or mass concentration, the preparation of a solution of a given molality is easy because it requires only a good scale; both solvent and solute are massed, rather than measured by volume.
  • Since the solution is very dilute, the molality is almost identical to the molarity of the solution, which is 1.3 M.
  • In this lesson, you will learn how molarity and molality differ.