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How to Find Moles

Introduction

In Chemistry the term "mole" is used widely. That is the basic thing you need to understand in order to solve the chemical equations and tasks properly. The word “mole” denotes the amount of substance which contains approximately 6.02214 * 10 ^ 23 of its elementary particles such as molecules, ions or atoms. That number is called the Avogadro's Constant. You need to know for sure the structure and the nature of the substance to tell definitely which types of elementary particles does it consist of. For example water consists of molecules, baking soda consists of ions and diamond consists of atoms.

To ease the calculations which presuppose operating the huge number which is called Avogadro's Constant it is often rounded to 6.022 * 10 ^ 23. Mole is usually measured in grams as science prefer using the metric system.

Do's

Basic principle of finding moles

To find the molar mass of a substance you need to remember a very simple rule: the mass of one mole of any substance is numerically equal to its molecular weight but is expressed in other terms. But what does define the molecular weight? Use the periodic table of the elements. With the help of the periodic table you will learn the atomic weight of each element. The atoms make up the molecules. After that you need to add up the atomic masses multiplied by the index of each item to find out the molar mass of the substance and get the answer.

Example (liquid or solid substances)

  • For example let us find the molar mass of the fertilizer widely used in agriculture called ammonium nitrate. The formula of this substance looks the following way: NH4NO3. How to determine what is its mole? First of all, make a note of the empirical (complete) formula of the substance. Since it looks like NH4NO3 you may easily see that it contains 2 atoms of nitrogen. That's why to make it look more comfortable you may write N2H4O3.
  • Calculate its molecular weight taking into account the index of each atom: 12 (nitrogen) * 2 + 1 (hydrogen) * 4 + 16 (oxygen) * 3 = 76 a. m. u. (atomic mass unit). Consequently its molar mass (i.e. the mass of one mole) also is 76 but its term is grams / mole. Now you have got the answer. One mole of ammonium nitrate weighs 76 grams.

Example (gas)

  • Suppose you were given this task. It is known that the mass of 179.2 liters of any gas is 352 grams. You need to determine how much weight one mole of gas. It is known that under normal conditions, one mole of any gas or gas mixture has a volume approximately equal to 22.4 liters. The given volume of gas 179.2 liters. Make a simple calculation: 179.2 / 22.4 = 8. Hence, the given volume contains 8 moles of gas.
  • Divide the mass of the gas by number of moles: 352/8 = 44. Therefore one mole of this gas weighs 44 grams. It is easy to find out that the given gas is carbon dioxide (CO2). 12 (carbon) *1 + 16 (oxygen) *2 = 44.

Don'ts

  • Don't hesitate to use the periodic table of elements to find the atomic weight.
  • Don't forget to take into account the indexes. Missing the indexes is the most common mistake among the students.
  • Don't neglect making the reversed calculation to check the result.

Video

Conclusion

If you still find it difficult you may watch the video. Try to solve a couple of tasks by yourself just to get involved and make it clear how to calculate moles. You may also ask your teacher to help you. A good idea is to read various textbooks and choose the clearest explanation.

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