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CHEMICAL ARITHMETIC

INTRODUCTION TO THE CHEMICAL CALCULATIONS

The stoichiometry is a part of chemistry which studies the relationships between the masses and volumes substances involved in chemical reactions. This issue involves the development and troubleshooting. To solve these problems, we need to know some basic concepts and applications.

The purpose of this lesson is to understand these fundamental concepts are: atomic weight, gram-atom, molecular weight, mol, equivalent weight of an element and a compound.

ATOMIC WEIGHT OF A CHEMICAL ELEMENT .- (P.at.) Since the chemical elements are usually a mixture of isotopes, is that we must understand that all the atoms that constitute not have the same mass and therefore the same weight, but we all know that each chemical element corresponds to a particular atomic weight, which is nothing but the average weight of its isotopes taking into account the percentage of abundance in nature.

According to this idea the atomic weight of an element could be determined by the following formula, if known masses of each of its isotopes and the percentages of abundance of each:

However, in all periodic tables, which recorded the characteristics of the different chemical elements are the relative atomic weights of chemical elements that were established on the basis of comparison to a carbon isotope is carbon-12.

The relative atomic weight of an item can be defined as the weight of the element that is to compare it to the twelfth carbon-12 (C-12) which by convention is assigned weight 12.0000 amu (atomic mass units).

Each chemical element is characterized by a specific atomic weight. H = 1.008

.........................
Cl = 35.453 C = 12.011 ............. ........... Na = 22.9897 15.9994 .....................
O = S = 32.06 N
Fe = 14.0067 = 55.847 .....................

gram-atom .- (At-g) Knowing the relative atomic weights The items we can determine the weights of individual atoms in grams, but such a determination was possible when scientists discovered that a number of grams of each component equal to their atomic weights, there is always the same number of atoms (6.023 x 1023).

Gram atom can be defined as an amount of grams of an element capable of containing 6.023 x 1023 atoms of the same. In other words we can say that an atom-gram of an element is equal to its atomic weight expressed in grams.

Example: If the P.at.

H = 1.008 amu (hydrogen 1átomo weight)
then:
1-g at H = 1.008 g (6.023 weight x10 23 hydrogen atoms)

If P.at. C = 12.011 amu (mass of 1 carbon át.de)
then:
1 at-g C = 12.011 g (6.023 weight x10 23 carbon atoms)

PROBLEM .- What is the weight in grams of one atom of hydrogen?

We rounded the atomic weight of hydrogen is 1 amu, then an AT-g it weighs 1g, but we also know one gram of hydrogen contains 6.023 x 1023 atoms. Taking into account these ideas:

BUILDING LONG-TERM MEMORY (41)

1. Calculate the atomic weight of silicon isotopes knowing that form and percentages of abundance are: Si-28 = 92.27%, Si-29 = 4.68% and Si-30 = 3.05%. Rpta 28.107 .-
2. The atomic weight of carbon is 12.011, if it consists of two isotopes, one of which is the C-12 where the percentage of abundance is 98.9%. What is the mass of other isotope? Rpta .- 13
3. determine the atomic weight of chlorine knowing that consists of two isotopes whose abundance percentages are: Cl-35 = 75.4% and Cl-37 = 24.6%. 35.492 .- Rpta
4. How many grams of sulfur atoms are in 100 g of the substance? Rpta .- 3.12
5. How many grams are in 2.75 at-g of iron? Rpta 154g .-
6. What is the mass in grams of one atom of lead? Rpta .- 3.44 x 10-24g
7. gram How many atoms are in 5 x 1030 atoms of sodium? Rpta .- 8 x 107
8. Taking into account the density of Ni is 8.9 g / cc. Calculate the volume of an atom of that element. Rpta .- 1.09 x 10 -23 cc.
9. Calculate the number of grams of Fe atoms that are in the same block of 8 x 2.5 x 10 cm. The density of iron is 7.9 g / cc. Rpta .- 28.2 at-g
10. It has 6.56 x 1023 atoms of lead and knowing that its density is 11.3 g / cc. What is the volume occupied? Rpta .- 2cc.
11. An analysis shows that chlorophyll is 2.68% Mg. How many Mg atoms are in 5 g of chlorophyll? Rpta .- 3.31 x10 19
12. Which of the following amounts contains more atoms, 13.4 g of Fe, 0.22 g-at Fe or 1.566 x 1023 atoms ? Rpta 1.566 .- 23 x 10
    Note: Please note that responses are not correct problems and in some exponential numbers the exponent is not well written.

MOLECULAR WEIGHT .- (PM) As we have seen in a previous lesson, a molecule can be formed by one or more identical or different atoms bonded together by covalent bonds. If we know the atomic weights of its elements, we can determine the weight of the molecule by adding the weights of the constituent atoms.

However, not all compounds are molecular compounds that we know, there are other are ionic compounds and as such their structural unit is the molecule but the ion. In the case of these compounds can not speak of molecular weight, it comes to weight formula weight formula or minimum (PF) is calculated as the molecular weight.

MOL .- It is the SI unit of amount of chemical substance which is defined as the amount of substance that contains as many elementary entities (atoms, molecules, ions, etc.) As there are atoms in 12 , 0000g of C-12 (6.023 x 1023).

If we know the molecular weight or formula weight of a substance can determine how much it weighs a gram-molecule or mole of the same.

Example: H2SO4 PM

= 98UM = weight of 1 molecule of sulfuric acid.
then:
1 mol of H2SO4 = 98 g = weight of 6.023 x10 23 H2SO4 molecules

PM 44uma = CO2 = weight of 1 molecule of CO2
then:
1 mol of CO2 = 44 g = weight 6.022 x 10 23 molecules of CO2 .

One mole of any substance contains 6.023 x10 23 molecules or so.

BUILDING LONG-TERM MEMORY (42)

1. How many moles are in 250g H3PO4? Rpta .- 2.55
2. How many molecules are in 0.75 mol of HCl? Rpta .- 4.517 x10 23
3. What is the mass in grams of 2.007 x 1022 molecules of KMnO4? Rpta .- 5.264 g
4. What is the mass in grams of one molecule of oxygen? Rpta .- 5.313 x10 -23
5. How weigh 0.8 mol of H2SO4 Rpta .- 78.4 g
6. How many moles equivalent to 3 x 10 20 molecules Rpta .- 4.982 x 10 -2
7. We
   0.15 moles of P4 a) How many molecules of P4 is? b) How many P atoms are there? Rpta .- a) 9x1022, b) 3.6 x10 23.
8. How many gram of oxygen atoms are in 0.15 moles of Ba (NO3) 2? Rpta .- 0.90.
9. How many atoms of nitrogen are in 100g of ammonium phosphate (NH4) 3PO4? Rpta .- 1.2127 x10 24
10. How many atoms carbon are in 1.08 g of quinine, C20H24N2O2? Rpta .- 4.015 x10 22
11. A drop of water is approximately equal to 0.05 ml. The density of water at room temperature is approximately 1g/ml. How many molecules of H2O are in a drop of water? 21 .- 2x10 Rpta
    Note: Please note that responses are not correct problems and in some exponential numbers the exponent is not well written.
    

EQUIVALENT WEIGHTS .- Also called gram-equivalent, are the weights of chemicals are equivalent for the purposes of reaction. For example, a gram-equivalent of ferric iron (18.67 g) reacts completely with an equivalent grams of oxygen (8g) to form ferric oxide, in the same direction, a gram-equivalent of an acid reacts completely with a gram-equivalent of hydroxide, or a gram-equivalent of an oxidizing agent reacts completely with a gram-equivalent of a reducing agent.

know the equivalent weights of acids, bases, oxidizing and reducing us to facilitate the realization of many chemical calculations.

GRAM EQUIVALENT OF A CHEMICAL ELEMENT .- (Eq-g) is defined as the weight of the element capable of combining with oxygen 8g or able to move 1 g of hydrogen from its compounds.
gram equivalents of a compound (Eq-g) .- Here are the formulas that allow you to determine the weight of one gram-equivalent of hydroxide, acid salt, an oxidizing agent and a reducing agent.

Eq-g. A HYDROXIDE .- The weight of the hydroxide can consume one mole of positive hydrogen ions or protons (H +). You can find this:

Eq-g. AN ACID .- The weight of the acid capable of producing one mole of positive hydrogen ions or protons (H +). You can find this:

Eq-g. A SAL :

BUILDING LONG-TERM MEMORY (43)

1. The analysis of a sample of a compound shows that it contains 11.33 g and 4.87 g Fe O. What is the equivalent weight of Fe and valence corresponding to that item? Rpta .- 18.6 and 3.
2. What is the equivalent weight of the phosphorus in the P4H10? Rpta .- 6.20.
3. Determine how much they weigh 2.5 gram equivalents of FeCl3? Rpta .- 134.166.
4. How many grams of sulfuric acid equivalent exist in 70g of the substance? Rpta .- 1.428.
5. Where there is a greater amount of substance "in 0.9 gram equivalent of Ca (OH) 2 or 0.7 eq-g of HNO3? Rpta .- 0.7-g eq.
6. A sample of 4.025 g of a metal displaces H 0.2016 g of hydrochloric acid, if the metal valence is 2, what is the equivalent weight of metal and what is its atomic weight? Rpta .- 20.12 and 40.24.
7. In redox reactions the oxidizing agent potassium dichromate, K2Cr2O7 becomes chromium chloride (III), CrCl3, determine how much it weighs one gram equivalent of potassium dichromate. Rpta .- 98g.
8. How weighing 2.5 gram equivalents of phosphoric acid, H3PO4? Rpta .- 32.66.
9. equivalents How many grams are in 250g of aluminum hydroxide, Al (OH) 3? Rpta .- 9.62.
10. What weight of Na2CO3 is needed to neutralize 4.98 g of HCl, knowing that the reaction is: Na2CO3 + 2 HCl ------- 2 NaCl + H2O + CO2. Rpta .- 7.11 g.
    

proposed project O

Find out how you can calculate the molecular dimensions experimentally or how you can determine about the Avogadro number (6.023 X1023 .) You must present the experiment with your peers.