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Mass Relationships in Chemical Reactions

Valentina Marotti

April 2015

BLock 7

AMOUNT OF REACTANTS AND PRODUCTS

• Used to calculate how much product will be formed from specific amounts of starting materials or how much staring material must be used to obtain a specific amount of product.

Steps

1. Write balanced chemical equation.

2. Convert quantities of known substance into moles.

3. Use coefficients in balanced equation to calculate the number of moles of the sought quantity.

4. Convert moles of sought quantity into desired unit.

CHEMICAL REACTION AND CHEMICAL EQUATIONS

EXPERIMENTAL DETERMINATION OF EMPIRICAL FORMULAS

• Shows the ratio of the number of the atoms in a molecule.

Determination of Molecular Formulas

• Is the sum of atomic masses in a formula.

• Lists all the atoms in a molecule.

AVOGADRO´S NUMBER AND MOLAR MASS OF ELEMENT

(Lorenzo Romano Amedeo Carlo Avogadro di Quaregua e di Cerreto)

• A chemical reaction is when a substance is changed into one or more new substances. It uses chemical symbols to show what happens during a chemical reaction.

• Reactants are the starting materials in a chemical reaction.

• Products are the substance formed as a result of a chemical equation.

• The arrow  means “to yield”.

How to balance chemical equations - task analysis

1. Identify all elements in the reactants side and in the products side.

2. Begin balancing by trying different coefficients.

3. Balance elements that do not have the same amount of elements.

4. Add coefficients

• Chemist measure atoms and molecule in moles (unit only available for chemistry).

• Mole is a unit used to count number of particles.

• Avogadro´s Number = 6.022 x 10 ^23  named after Amedeo Avogadro (he was Italian, so he was very cool).

• Avogadro´s Number: the number of particles in a mole. Is used to convert mass in grams to atomic mass units or vice versa.

• Molar Mass: Mass of 1 mole of unit of a substance. Same value as Atomic Mass.

LIMITING REAGENTS

THE MASS SPECTROMETER

• Limiting reagents are the reactant used first in a chemical reaction.

• Excess reagents are the reactants present in quantities greater than necessary to react with the quantity of the limiting reagent.

• The most accurate method for determining atomic and molecular masses. In other words measures the mass of the atom.

MOLAR MASS

Mass Spectrometer

MOLECULAR MASS

• Molecular Mass: the sum of atomic masses in a molecule.

• With the molecular mass we can determine the molar mass of a molecule or compound. We can calculate the number of moles and individual atoms in a given quantity of a compound.

• Molar Mass (g) = Molecular Mass (amu)

Diagram

Exercises Balancing Equations

Empirical formulas vs. Molecular formulas

BIBLIOGRAPHY

REACTION YIELD

ATOMIC MASS

• Theoretical yield: the amount of product that would result is all the limiting reagent reacted.

• Actual yield: is the amount of product actually obtained from a reaction.

• Percent yield: the proportion of the actual yield to the theoretical yield.

PERCENT COMPOSITION

Chang, R., & Glodsby, K. (2013). Chemistry, Eleventh Edition (Eleventh Edition ed., p. 1080). New York: McGraw-Hill Companies.

Maria Elena, V. (2014). Chemistry Class.

Pareja, E. (2015). Chemistry Class.

• The mass of an atom depends on the number of protons, neutrons, and electrons it contains.

• Atomic Mass is the mass of the atom in atomic mass units or also called amu; located above the element symbol in the periodic table.

• Example: C = 12amu

• Average Atomic Mass. Average mass of naturally occurring isotopes. Example: Carbon- 12 (98.90 percent abundance, 12amu) Carbon-13 (1.10 percent abundance, 13amu). To calculate the average atomic mass we must multiply the percent and the amu of each. Then add these values and divide them by 100, the answer id the average atomic mass for Carbon.

Atomic Mass of Carbon

• Percent composition of mass is the percent by mass of each element in a compound.

• Used to determine the purity in a compound by calculating the percent of the total mass of the compound and comparing the result to the present composition.

• With the percent composition we can determine the empirical formula.

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