Kaplan MCAT: Gen Chem 2

amount of moles of a substance (mol)
Weight of sample(g)/Molar weight (g/mol)
How many mol. in 9.52 g of MgCl2?
MW=95.21
–> 9.52/95.21= 0.10 mol
What is the calculation for Equivalents?
Weight of Compound/Gram Equivalent weight (eg) HCl can dissociate into 1 H+ ion– 1 eq H2SO4 can donate 2 H+ ions–2 eq
Percent Composition

{ Mass of (X) in Formula   }*100%

{Formula weight of compound }

What is the percent composition of chromium in K2Cr2O7?

Formula weight of compound is 2(39)+2(52)+7(16)=294 g/mol

 

%comp Cr= 2(52g/mol)*100%

                 294(g/mol)

 

= .354*100%= 35.4%

What is a combination reaction?

2 or more reactants form 1 product

 

S (s) + O2 (g) —> SO2 (g)

Decomposition Rxn?

Compound breaks into 2 or more substances (usually as a result of heating or electrolysis).

 

2HgO (s) —Δ—> 2Hg (l) + O2 (g)

Single Displacement Reaction?

Occur when an atom (or ion) of one compound is replaced by an atom of another element

 

Often classified as redox reactions, further discussed in ch.11

 

 

Zn (s) + CuSO4 (aq) —> Cu(s) + ZnSO4(aq)

Double Displacement reaction?

also called metathesis reaction, elements from 2 different compounds displace each other to form 2 new compounds. Occurs when 1 of the products is removed from the solution as a precipitate or gas, or when two of the original species combine to form a weak electrolyte that reamains undissociated in sol’n.

 

ex.

 

CaCl2(aq) + 2 AgNO3(aq)–> Ca(NO3)2 (aq)+2AgCl(s)

Guidelines for balancing chemical equations?

  1. ID reactants and products of the reaction if they are not completely specified. Write formulas of the reactants on left side of the arrow and the formulas of the products on the right side.
  2. Balance the element that has the least amount of molecules.
  3. Continue balancing elements as they appear. Balance the last element  that appears in the most quantity.
  4. As necessary, multiply all coefficients on both sides of the rxn. by the same amount in order to clear fractional coefficients.

Combustion Rxn?

Type of combination where the substance is burned in the presence of gas.

 

3Mg (s) + N2 (g) –> Mg3N2 (s)

Corrosion Rxn?

Metals deteriorate in corrosion rxns. when a liquid or gas (usually Oxygen) chemically attacks the surface of the metal

 

Eg. The formation of Rust (Iron Oxide) from iron

 

4 Fe (s) + 3O2 (g) –> 2Fe2O3 (s)

 

 

BUT formation of CO2 from Oxygen attacking graphite is NOT corrosion.  Graphite is solid carbon, which is a nonmetal.

Formation Rxn?
all reactants must be in their standard elemental state, eg. no molecules.
Combination rxn.
reducing vs. oxidizing agents?
reducing agent: is oxidized, reduces another chemical

oxidizing agent: is reduced, oxidizes another chemical

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