As described in the previous section, sodium chloride is quite soluble in water. At 25 ˚C (about room temperature), 359 grams of sodium chloride will dissolve in one liter of water. If you were to add more sodium chloride to the solution, it would not dissolve, because a given volume of water can only dissolve, disperse and stabilize a fixed amount of solute (the stuff that dissolves). This amount is different for every compound and it depends on the structure of the particular compound and how that structure interacts with the solvation shell. When a substance is dissolved in water to the point that no more will go into solution, we say the solution is saturated. For most compounds, heating the solution will allow more of the substance to dissolve, hence it is important to note the temperature when you are speaking of the solubility of a particular compound. If we had a saturated solution of sodium chloride at 25 ˚C, we could quote the concentration as 359 grams/L, but because we know the molar mass of sodium chloride (58.44 grams/mole), we could also express our concentration as: \[\left ( \frac{(359\, g)\times \frac{1\, mole}{58.44\, g}}{1\, L} \right )=6.14\, moles/L \nonumber \] In chemistry, the units of moles/L are called molarity, with the abbreviation M. Thus we could say that our saturated solution of sodium chloride was 6.14 molar, or 6.14 M. The advantage of expressing concentrations in terms of molarity is that these solutions can now be used in chemical reactions of known stoichiometry because any volume of the solution corresponds directly to a known number of moles of a particular compound. For example, the molar mass of potassium bromide is 119.0 g/mole. If we dissolved 119.0 grams of KBr in 1.000 L of water, the concentration would be 1.000 mole/L, or 1.000 M. If we now took half o this solution (0.500 L) we know that we would also have 0.500 moles of KBr. We can determine the concentration of a solution using the problem-solving algorithm we introduced back in Chapter 1. For example, if you wan to find the molarity of a solution containing 42.8 grams of KBr in 1.00 L of water, you would identify the given and 42.8 g, your ratio is the molar mass (119 g/mole) and you want to find molarity (or moles/L). Remembering to set the equation up so that the units of given appear in the denominator of the ratio, the number of moles is: \[42.8\, g)\times \left ( \frac{1\, mole}{119\, g} \right )=0.360\, moles \nonumber \] and, the molarity is: \[\left ( \frac{0.360\, mole}{1.00\, L} \right )=0.360\, moles/L\; or\; 0.360\, M \nonumber \] When you become comfortable with the simple two-step method, you can combine steps and simply divide your given mass by the given volume to get the result directly. Thus, if you had 1.73 grams of KBr in 0.0230 L of water, your concentration would be: \[\left ( \frac{(1.73\, g)\times \frac{1\, mole}{119\, g}}{0.0230\, L} \right )=6.32\, moles/L\; or\; 0.632\, M \nonumber \] We can also solve these problems backwards, that is, convert molarity into mass. For example; determine the number of grams of KBr that are present in 72.5 mL of a 1.05 M solution of KBr. Here we are given a volume of 0.0725 L and our ratio is the molarity, or (1.05 moles/L). We first solve for moles, \[0.0725\, L\times \left ( \frac{1.05\, mole}{1.00\, L} \right )=0.0761\, moles \nonumber \] and then convert to mass using: \[0.0761\, moles\times \left ( \frac{119\, grams}{1\, mole} \right )=9.06\, grams\, of\, KBr \nonumber \]
A sample of 12.7 grams of sodium sulfate (Na2SO4) is dissolved in 672 mL of distilled water.
Calculate the mass of sodium chloride required to make 125.0 mL of a 0.470 M NaCl solution. If you dissolve 5.8g of NaCl in water and then dilute to a total of 100.0 mL, what will be the molar concentration of the resulting sodium chloride solution? Definitions Solute-the substance being dissolved Solvent-the substance doing the dissolving (the larger amount) Solution- a homogeneous mixture of the solute and the solvent Solution= solvent + solute Aqueous (aq)= water solution Tincture= alcohol solution Amalgam= Mercury solution Molarity (M)- is the molar concentration of a solution measured in moles of solute per liter of solution.  The molarity definition is based on the volume of the solution, NOT the volume of water. Vocab. Lesson Incorrect= The solution is 5.0 Molarity. Correct= The solution is 5.0 Molar. Example Problems Level 1- Given moles and liters Determine the molarity when 3.0 moles of sucrose are dissolved to make 2.0 liters of solution.
Level 2- Given Molarity and liters of solution Determine the number of moles of salt dissolved in 5.0 liters of a 0.50M solution of salt water.
cross multiply, X= 2.5 mols Level 3- Given grams (instead of moles) and liters of solution Determine the molarity when 117g of NaCl are dissolved to make 0.500 liters of solution. 1st convert to moles, 2nd plug into the molarity equation 117g NaCl( 1mol/58.5g)= 2.00mol NaCl
Level 4-Given grams (instead of moles) and milliliters of solution (instead of liters) Determine the molarity when 55.5g of CaCl2 are dissolved to make 250.mL of solution. 1st convert to moles, 2nd convert to liters, 3rd plug into the molarity equation 55.5g CaCl2( 1mol/111g)= 0.500mol CaCl2 250.ml( 1L/1000mL) =0.250L
Past Regents Questions-Follow link to check the answers Jan 2003-44 What is the molarity of a solution of NaOH if 2 liters of the solution contains 4 moles of NaOH?
Jan. 04-41 What is the molarity of a solution containing 20 grams of NaOH in 500 milliliters of solution? (1) 1 M (2) 2 M (3) 0.04 M (4) 0.5 M Jan 2002-42 What is the molarity of a solution that contains 0.50 mole of NaOH in 0.50 liter of solution?
Aug. 2006-42 How many total moles of KNO3 must be dissolved in water to make 1.5 liters of a 2.0 M solution? (1) 0.50 mol (2) 2.0 mol (3) 3.0 mol (4) 1.3 mol Aug 2005- 41 What is the total number of moles of NaCl(s) needed to make 3.0 liters of a 2.0 M NaCl solution?(1) 1.0 mol (3) 6.0 mol (2) 0.70 mol (4) 8.0 mol June 2006- 16 Molarity is defined as the(1) moles of solute per kilogram of solvent(2) moles of solute per liter of solution(3) mass of a solution (4) volume of a solvent Aug 2008- 15 Which phrase describes the molarity of a solution?(1) liters of solute per mole of solution(2) liters of solution per mole of solution(3) moles of solute per liter of solution (4) moles of solution per liter of solution June 2009-46 Which sample of HCl(aq) contains the greatest number of moles of solute particles? June 2007- 13 A 3.0 M HCl(aq) solution contains a total of(1) 3.0 grams of HCl per liter of water(2) 3.0 grams of HCl per mole of solution(3) 3.0 moles of HCl per liter of solution (4) 3.0 moles of HCl per mole of water June 2010-14 The molarity of an aqueous solution of NaCl is defined as the(1) grams of NaCl per liter of water(2) grams of NaCl per liter of solution(3) moles of NaCl per liter of water (4) moles of NaCl per liter of solution Jan 2008- 15 Which unit can be used to express solution concentration?(1) J/mol (3) mol/L (2) L/mol (4) mol/s Jan 04-41 What is the Molarity of a solution containing 20 grams of NaOH in 500 milliliters of solution?(1) 1 M (3) 0.04 M (2) 2 M (4) 0.5 M Jan 2010-40 What is the molarity of 1.5 liters of an aqueous solution that contains 52 grams of lithium fluoride, LiF, (gram-formula mass =26 grams/mole)?(1) 1.3 M (3) 3.0 M (2) 2.0 M (4) 0.75 M on to ppm or Molality back to Math of Chemistry Links
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What is the molarity of a solution that has 29.2 5 g of NaCl dissolved to make 1.5 Litre of a solution?
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