Formulas, Equations, and Moles

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Formulas, Equations, and Moles

• Chapter 3

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Molar Mass

• The sum of the atomic masses for all the atoms
  represented in the chemical formula of a
  compound.
• AKA formula mass, molecular mass, molecular
  weight

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Mole (mol)

• Number of particles in atomic mass in grams of an
  element.
• Number of molecules/formula units in the molar
  mass in grams of a compound
• 6.02 x 1023 particles.

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Fill in the table for 1 mol of butyl mercaptan
(skunk odor)
CH3(CH2)3SH carbon hydrogen sulfur
Number of moles
Number of atoms or molecules
Molar mass
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• How many water molecules are in one drop of
  water? (One drop of water is 1/20 of a mL, and
  the density of water is 1.0 g/mL.)
• How many hydrogen atoms are in a drop of water?

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Percent Composition

• Nitrogen fixation in the root nodules of peas and
  other legumes occurs with a reaction involving a
  molybdenum containing enzyme named nitrogenase.
  This enzyme contains two Mo atoms per molecule
  and is 0.0872 Mo by mass. What is the molar
  mass of the enzyme?

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Empirical Formulas

• Simplest formula for a compound.
• Can be determined from percent composition data
  and combustion analysis.

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• An analysis of nicotine, a poisonous compound
  found in tobacco leaves, shows that it is
• 74.0 C,
• 8.65 H, and
• 17.35 N.
• Its molar mass is 162 g/mol. Determine the
  empirical and molecular formulas for nicotine.

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• A compound of Ca, C, N, and S was subjected to
  quantitative analysis and formula mass
  determination, and the following data were
  obtained. A 0.250 g sample was mixed with NaCO3
  to convert all of the Ca to 0.160 g of CaCO3. A
  0.115 g sample of the compound was carried
  through a series of reactions until all of its S
  was changed to 0.344g of BaSO4. A 0.712 g sample
  was processed to liberate all of its N as NH3,
  and 0.155 g NH3 was obtained. The formula mass
  was found to be 156. Determine the empirical and
  molecular formulas of this compound.

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Mass percent

• (mass part/mass whole) x 100()
• An aqueous solution of acetic acid is 20 percent
  by weight Acetic acid has a density of 1.0269
  g/mL. How many moles of acetic acid are
  contained in 35 mL of this solution?

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Molarity (M)

• Mole solute/ Liter solution
• 128 mL of 0.306 M Na2SO4 is mixed with 220 mL of
  0.827 M NaCl. Inventory the resulting molarities
  of the ions.

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• Dilution problem
• For molarity -- M1V1 M2V2
• A chemist wants to prepare 0.25 M HCl.
  Commercial HCl is 12.4M. How many mL of the
  concentrated acid does the chemist require to
  make up 1.50 L of the dilute acid?

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Stoichiometry

• Disulfur dichloride S2Cl2, is used to vulcanize
  rubber. It can be made by treating molten
  sulfur with gaseous chlorine
• S8(l) 4 Cl2(g) ? 4 S2Cl2(g)
• Calculate the mass of S2Cl2 that can be produced
  by reacting 15.0 g of S8 with excess chlorine gas.

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Limiting Reagent Stoichiometry

• Some of the acid in acid rain is produced by the
  following reaction
• 3NO2(g) H2O(l) ? 2HNO3(aq) NO(g)
• If a falling raindrop weighing 0.050 g comes into
  contact with 1.0 mg of NO2(g), how much HNO3 can
  be produced?

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• One of the steps in the commercial process for
  converting ammonia to nitric acid involves the
  conversion of NH3 to NO
• 4 NH3(g) 5 O2(g) ? 4 NO(g) 6 H2O(g)
• How many grams of NO form when 1.50 g of NH3
  reacts with 1.85 g of O2?
• Which reactant is the limiting reactant and which
  is the excess reactant?
• How much of the excess reactant remains after the
  limiting reactant is completely consumed?

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• The potassium salt of benzoic acid, potassium
  benzoate (KC7H5O2) can be made by the action of
  potassium permanganate on toluene (C7H8) as
  follows
• C7H8 2KMNO4 ? KC7H5O2 2MnO2 KOH H2O
• If the yield of potassium benzoate cannot
  realistically be expected to be more than 71,
  what is the minimum number of grams of toluene
  needed to achieve this yield while producing 11.5
  g of potassium benzoate?

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Solution Stoichiometry

• How many milliliters of 0.750 M Pb(NO3)2 solution
  are required to react completely with 1.00 L of
  2.25 M NaCl solution? (Double displacement
  reaction)

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• 581 mL of 0.342 M Barium nitrate Ba(NO3)2 are
  mixed with 264 mL of 0.631 M potassium sulfate
  (K2SO4). Write the molecular, ionic, and net
  ionic reactions for this reaction, calculate the
  mass of any precipitate produced and inventory
  the ions in solution after precipitation.

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• A noncarbonated soft drink contains an unknown
  amount of citric acid, H3C6H5O7. If 100.0 mL of
  the soft drink requires 33.51 mL of 0.01024 M
  NaOH to neutralize the citric acid completely,
  what is the concentration of citric acid in the
  soft drink in moles/liter (M)? In
  equivalents/liter (N)?

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• A 0.2857 gram sample of sodium phosphate is
  titrated by 48.22 mL of a sulfuric acid solution.
  What is the concentration of the sulfuric acid
  solution?
• What is the pH of the sulfuric acid solution? The
  pOH?

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• Suppose you are given a 4.554 g sample that is a
  mixture of oxalic acid, H2C2O4, and another solid
  that does not react with sodium hydroxide. If
  29.58 mL of 0.5501 M NaOH is required to titrate
  the oxalic acid in the 4.554 g sample, what is
  the weight percent of oxalic acid in the mixture?

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• A vitamin C tablet was analyzed to determine
  whether it did in fact contain, as the
  manufacturer claimed, 1.00 g of the vitamin. A
  tablet was dissolved in water to form a 100.00 mL
  solution, and a 10.00 mL sample was titrated with
  iodine (as KI3, potassium triiodide). It
  required 10.10 mL of 0.0521 M I3-1 to reach the
  stoichiometric point in the titration. Given
  that vitamin C, C6H8O6, is oxidized to
  dehydroascorbic acid, C6H6O6 and triiodide, I3-1,
  is oxidized to iodide, I-1, write a balanced
  equation for the reaction and determine whether
  the manufacturers claim is correct? (MW vitamin
  C 176 g/mol)

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• A 35.49 mL sample of 0.2430 M H2SO4 is mixed with
  65.33 mL of a 0.4199 M sample of KOH. Determine
  the concentration of all ions in solution, the
  pH, and pOH.

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• Some compounds can be decomposed quantitatively
  with water or acid to give known compounds.
  Suppose you have a 0.643 g sample of a compound
  known to be composed of C, H, Al, and Cl.
  Furthermore, you know that it is composed of some
  number of CH3 groups and chlorine atoms per
  aluminum atom. The formula could be written as
  (CH3)xAlCly. To find x and y you decompose the
  sample with acid in water. The CH3 portion is
  evolved as methane gas, CH4, and the chloride
  ions remain in the water. The chloride ions are
  precipitated as AgCl by adding AgNO3 to the
  solution. The data collected in the experiment
  are given here. What are the values of x and y?

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• (CH3) xAlCly ?? x CH4(g) Al3(aq) y Cl-(aq)
• 0.643g 0.222g ?
• AgNO3
• ?
• AgCl(s)
• 0.996g

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• The cancer chemotherapy agent, cisplatin, is made
  by the following reaction
• (NH4)2PtCl4(s) 2 NH3(aq) ? 2 NH4Cl(aq)
  Pt(NH3)2Cl2(s)
• If 15.5 g of (NH4)2PtCl4 is combined with 225mL
  of 0.75 M NH3 to make cisplatin,
• a. Which reactant is in excess and which is the
  limiting reactant?
• b. How many grams of cisplatin can be formed?
• c. After all of the limiting reactant has been
  consumed and the maximum quantity of cisplatin
  has been formed, how much of the other reactant
  remain?