Chapter 4: Stoichiometry (C9127316)

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1 Molecular weight

Molar mass is the mass in grams, of a mole of a substance, and has the units g/mol. See  for more information about a mole. Since an AMU is [mathjax]1/12th[/mathjax] of the mass of one carbon-12 atom, the mass of a single entire carbon-12 atom is [mathjax]12 AMU[/mathjax]. Remember that a mole is by definition the number of atoms in 12 grams of carbon-12. Therefore, a mole of carbon-12 is 12 grams. As the mass of carbon-12 is 12 AMU, the statement can be restated as “a mole of 12 AMU is 12 grams”, meaning [mathjax]N_{A}\times 12 AMU=12g[/mathjax], or reshuffling, [mathjax]AMU=\dfrac{1g}{N_{a}}[/mathjax]. Based on the formula, AMU can have the units g/mol. Converting grams to kilograms, [mathjax]AMU=\dfrac{1}{N_{A}} \div 1,000[/mathjax] kg. Demonstrating the inverse relationship between AMU and a mole (Avogadro’s number), [mathjax]\dfrac{1}{N_{a}} \div 1,000 = \dfrac{1}{6.02\times 10^23}\div 1,000=1.66\times 10^-27 =AMU[/mathjax]: quod erat demonstrandum. Given molar mass has the g/mol unit too, AMU and molar mass are thus interchangeable. AMU, also referred to as atomic mass, atomic weight, can be found on the periodic table, and since is equivalent to molar mass, can be used in lieu of molar mass for the [mathjax]n=\dfrac{m}{M}[/mathjax] formula.

Molecular mass (also known as molecular weight) refers to the mass of not just a single atom, but a molecule, which is a collection of atoms.

Formative learning activityMaps to RK4.1
What is molecular weight?

2 Empirical, molecular formula

Compounds can be described with an empirical formula, which provides the simplest [whole number] ratio of the different types of atoms, permitted as there are no distinct groups in compounds. This is in contrast with molecular formula, which provides exact numbers of each atoms, required for molecules which express exact numbers of atoms in each group. For example, glucose is a molecule with the molecular formula [mathjax]\ce{C6H12O6}[/mathjax], which would be underrepresented if expressed via the empirical formulas [mathjax]\ce{CH2O}[/mathjax].

Formative learning activityMaps to RK4.2
What is empirical formula? What is molecular formula, and how is this distinct from empirical formula?

3 Units in chemistry

SI units is the modern form of the metric system. Metric system is an international decimal system. SI units are built around seven base units, from which all other units are constructed, of which six include:

  • Meter (m), which measures length
  • Second (s), which measures time
  • Kilogram (kg), which measures mass
  • Kelvin (K), which measures temperature
  • Mole (mol), which measures amount
  • Coulomb (C), which measures electrical charge

4 Percent mass composition

Percent mass composition is the fraction of the atomic mass of one element, to the total mass of the compound. The total mass is calculated by adding the atomic masses of all the atoms in the compound. This can be found from the empirical formula. For example, the percent mass of carbon in [mathjax]\ce{CH2O}[/mathjax] is [mathjax]\dfrac{12}{12+2(1)+16}=40%[/mathjax]. Evidently, atomic mass could be determined from the molecular formula, but this would increase the complexity of the calculation.

Formative learning activityMaps to RK4.4
What is percent mass composition?

5 Moles, Avogadro's

“Mole is that thing you whack in an arcade,” Mandy said.

“It’s that black dot that grows on your face, and no amount of make-up can really remove it,” Jamie giggled.

“I don’t have one on my face,” Mandy replied, “but…”

Mole is a constant, defined as the number of atoms in 12 grams of carbon-12. This number is known as Avogadro’s number ([mathjax]N_{A}[/mathjax]), namely [mathjax]6.02\times 10^23[/mathjax] atoms. To convert from mass to moles, use [mathjax]n=\dfrac{m}{M}[/mathjax], where [mathjax]n[/mathjax] is the number of moles, [mathjax]m[/mathjax] is the mass in grams, and [mathjax]M[/mathjax] is the molar mass.

Formative learning activityMaps to RK4.5
What are moles? What is Avogadro's number, and how does this relate to moles?

6 Definition of density

Density is the mass per unit volume. Less dense fluids float on top of more dense fluids if they don’t mix.

Formative learning activityMaps to RK4.6
What is the definition of density?

7 Oxidation number

Oxidation number of a central atom, is the charge it would have if all the atoms it binds to were removed. For example, the oxidation number of iron in [mathjax]\ce{Fe2O3}[/mathjax] is [mathjax]+3[/mathjax], and the oxidation number of oxygen is [mathjax]-2[/mathjax].

Formative learning activityMaps to RK4.7
What are oxidation numbers?

8 Chemical equations

Chemical equation is the symbolic representation of a chemical reaction, where reactants are on the left hand side, and the products are on the right hand side. Arrows going only in the forward direction show a net forward reaction. Arrows going in both directions indicate an equilibrium.

Limiting reagent is the reactant that is entirely consumed before others in a chemical reaction, thereby limiting the reaction, as the reaction cannot proceed without it.

The theoretical yield is the amount of product obtained in a chemical reaction if it ran to completion, without competing reactions. Competing reaction are other reactions that can occur, which use up the reactants. The percentage yield is the actual yield divided by the theoretical yield.

Formative learning activityMaps to RK4.8
What are chemical equations?

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Pre-med science (MED5118352)


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Chapter 4: Stoichiometry - General chemistry - Pre-med science - MR. SHUM'S CLASSROOM