Chemistry lessons are filled with various experiences and an abundance of tasks that will have to be solved, regardless of your desire and mood. Solving organic chemistry problems or inorganic chemistry requires a lot of time and good knowledge of the entire course, attentiveness, serious and detailed work. However, having studied the secrets of our experts, it will become easier to solve chemistry problems.
Today we will look at one of the tasks that students face most often. We will teach you how to find the molar mass of NaOH. Before you start studying the examples, plunge into a little theory. Without it, you will not understand what a chemical formula is and how a chemical reaction occurs—stock up on patience and strength. In return, you will receive the knowledge that teachers do not give you.
Mole: What Is This?
The concept "mole" appeared not so long ago and has no physical meaning. This is an artificially introduced value. For example, in old textbooks, instead of the concept "mole," the concept "gram-molecule" was used. A mole is a substance that contains several molecules (particles, ions, atoms) equal to Avogadro's number NA=6*1023.
The number 6.02 * 1023 is called the Avogadro number in honor of the Italian chemist Amedeo Avogadro. Why is this number chosen to define the mole? The fact is that exactly as many atoms are contained in 12 g of the carbon isotope 12C. The same isotopes are used to select the atomic mass unit.
Avogadro's number is also called Avogadro's constant, and scientists refer to this with a special symbol NA. This constant has a dimension - pieces per mole or mole-1. Thus:
For approximate calculations, Avogadro's number can be rounded up to 6*1023. Knowing Avogadro's constant, we can express any amount of a substance in a mole. If a substance contains N molecules, then the amount of substance (denoted by the Greek letter ν) is equal to ν=N/NA. The formula works the same way in reverse. Knowing the amount of substance in a mole, you can find the number of molecules: N=ν*NA. For example:
- 1 mol of copper contains NA = 6 * 1023 atoms.
- 1 mol of sodium chloride - NA = 6 * 1023 NaCl molecules.
- 1 mol of sodium ions - NA = 6 * 1023 ions.
Molar And Molecular Weight
You can apply Avogadro's law only to gaseous substances. However, chemists need to know how many molecules there are in liquids and solids. Therefore, to extract the number of molecules in substances, scientists have introduced a molar mass value. This value means the mass of one mole of a substance. Molar mass stands for letter M, and it is numerically equal to the relative molecular weight. Also, there is a formula weight M = m/v, which you should use in all chemical tasks for calculating molar mass. The letter m means the mass of a substance, and v is the number of substances.
Molecular mass refers to the mass of a molecule expressed in atomic mass units. Molecular weight is numerically equal to the molar weight. In chemistry, there is an absolute molecular weight and a relative molecular weight. Scientists most often use a dimensionless value (relative molecular weight) since experiments show how many times the mass of a molecule exceeds 1/12 of a carbon atom mass. It is customary to denote molecular weight by the symbol Mr.
However, one should clearly understand the difference between molar mass and molecular mass, realizing that they are equal only numerically and differ in dimension. The molecular weights of complex molecules can be determined by adding up their constituent elements' molecular weights. For example, the molecular weight of water (H2O) is MH2O = 2 MH + MO = 211 + 16 = 18 amu.
NaOH: What is This?
Molecular formula: NaOH.
Molar and molecular weight of a chemical: 39,997 g/mol.
The number of atoms: 3 - Na, O, H.
Melting point: 323 °C.
Boiling point: 1403 °C.
Solubility in water: 108,7g/100 ml.
Limiting concentration: 0,5 mg/m³.
This reagent, the most common alkali, is better known under the name caustic soda. Based on the name, it is clear that the substance is dangerous. Therefore, you must handle it with care. Sodium hydroxide is a colorless crystalline mass and has a corrosive function. The sodium hydroxide solution is capable of corroding organic materials and certain metals. On contact with zinc, lead, aluminum, tin, and their alloys, hydrogen, an explosive gas, is released. Do not allow caustic soda to come into contact with ammonia, and it is fire hazardous. Sodium hydroxide is used to neutralize acids and acidic oxides. It also plays the role of a catalyst in several chemical reactions. It is used in the titration, etching of aluminum, and in the manufacture of pure metals.
How To Find Molar Mass of NaOH Solution?
To calculate the molar mass of a chemical to adhere to the following algorithm:
1. Prepare the periodic table. It may be needed to determine the valence and atomic masses of chemical elements.
2. Correctly draw up a formula, using knowledge about the main classes of inorganic chemical compounds and their properties. Also, information from the periodic table. For example:
- caustic soda - NaOH;
- potassium hydroxide - KOH;
- carbon dioxide - CO2;
- hydrochloric acid - HCl;
- sulfuric acid - H2SO4;
- calcium chloride - CaCL2;
- aluminum hydroxide - Al(OH)3.
First of all, when drawing up compound formulas, it is necessary to remember the valence of the elements of which they are composed.
3. Determine the molecular weight and molar mass of NaOH. We again take the weight of atoms in the cell of a chemical element in the periodic table:
Molecular mass = Mr [NaOH] = Ar [Na] + Ar [O] + Ar [H] = 22.98976928 + 15.9994 + 1.00794 = 39.99710928
Molar mass = Mr [NaOH]: 1000 = 39.99710928 : 1000 = 0.04 kg/mol
As you can see from the example, to perform the necessary calculations, it is enough to add up the molar mass of Na, O, H - elements that make up the substance.
4. Determine the mass of one molecule of each of these compounds using the Avogadro number:
m(NaOH) = Mr(NaOH)/NA= 39/6,02*1023 = 6,4*1023 g
By applying the algorithm, you will quickly find the number of moles.
Example How to Solve a Task
Task: What mass of sodium hydroxide (NaOH) contains the same amount of equivalents as 140 grams of potassium hydroxide (KOH)?
Calculate the molar mass of potassium hydroxide equivalent (KOH) using the formula:
We get: Meqv(KOH)=M(KOH)* feqv
Therefore, 140 g of potassium hydroxide (KOH) contains 140/56 = 2,5 equivalents.
Calculate the molar mass of the equivalent of sodium hydroxide (NaOH) by the formula:
We get: Meqv(NaOH)=40 * 1=40 (g/mol)
Hence: 2.5 equivalents are 40 g / mol *2.5 mol = 100 (g).
Answer: The m NaOH is 100 grams.
How is NaOH Prepared?
Sodium hydroxide is produced industrially by electrochemical and chemical methods. All industrially produced sodium hydroxide is obtained by electrolysis of an aqueous solution of NaCl. The process also produces gaseous chlorine and hydrogen (this is a very strong base):
2NaCl + 2H2O → 2NaOH + Cl2 + H2
The electrochemical method uses solid cathode electrolysis and liquid mercury cathode electrolysis. The mercury method has significant drawbacks in environmental impact, although it is much simpler in technical implementation. The membrane method for producing sodium hydroxide is the most efficient and the most complex. Membrane electrolysis provides the purest sodium hydroxide.
The most used chemical methods for producing sodium hydroxide are lime and ferritic. The lime method consists of the interaction of a solution of soda ash with slaked lime. The ferritic method requires the use of soda ash and iron oxide. The process takes place in two stages. When a mixture of soda is calcined with iron oxide, sodium ferrite is formed. The resulting sodium hydroxide solution is evaporated, and a reliable product is obtained from one stripped-off solution. Iron oxide is returned to the production cycle.