Chemistry
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Amount of Substance

Amount of Substance

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Atoms are incredibly small, so much so that scientists can't count them individually. To give you an idea of how small they are, 500,000 carbon atoms stacked together are as wide as a single strand of human hair! In this article, we'll explore the amount of substance and the important concepts related to it. You'll discover Avogadro's constant (L) and how to calculate the number of moles (n). We'll also talk about relative atomic mass (Ar) and why we use the unified atomic mass unit (u). Additionally, we'll discuss Percentage Yield and why actual yield is lower than theoretical yield. Finally, you'll learn about the ideal gas equation and kinetic gas theory. If you want to learn more about any of these topics, just click on the linked topics.

What is the amount of substance?

The amount of substance, also known as the chemical amount, refers to the number of particles or elementary entities present in a sample. The mole is the unit used to measure the amount of substance. An elementary entity is the smallest amount of a substance that can exist, and it can be an atom, molecule, ion, or electron. It is important to specify the type of elementary entity when discussing the amount of substance. For instance, the elementary entity for oxygen is not just the oxygen atom (O), but also molecular oxygen (O2), since two oxygen atoms combine to form a molecule. When talking about the amount of substance of covalent compounds, we refer to their molecular formula, while for ionic compounds, we refer to their formula units.

Oxygen is a diatomic particle
Oxygen is a diatomic particle

What is the mole?

A mole is a precise quantity that refers to 602 hexillion things, which is equivalent to 602,200,000,000,000,000,000,000. We write it as  for short. This value is also known as Avogadro's constant (L) and is named after an Italian scientist named Amedeo Avogadro. He discovered that gases contain the same number of molecules in equal volumes under the same conditions, which led to the calculation of Avogadro's constant.

The exact number of atoms in 12 grams of carbon-12 isotope is equal to Avogadro's constant (L), which is . The unit used to measure this quantity is per mole. The mole is the SI unit for the amount of substance.

To calculate the number of moles, we use the formula:orn: number of molesm: mass in gramsM: molar mass (the mass of 1 mole in grams)

In one mole of any substance, there are exactly  elementary entities, or Avogadro's constant. For instance, if one mole of a substance weighs 80 g and you have 10 g of it, you can calculate the number of moles using the formula:

number of moles = mass in grams/molar mass

Thus, the number of moles of the substance you have is 0.125 moles (10 g/80 g/mol).

Why is the mole important?

The mole is a crucial concept in the context of chemical reactions. When we write a chemical equation, we need to specify the number of molecules of each reactant and product involved. For example, in the equation 2H2 + O2 → 2H2O, we need two times as many molecules of hydrogen as we need oxygen to ensure that the reaction proceeds as intended. If we had 1 mole of hydrogen and 1 mole of oxygen, they would have the same number of molecules, Avogadro's constant. To ensure that we have twice as many hydrogen molecules, we need two moles of hydrogen.

Thus, we can also write the above equation as "2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water." The mole is useful because it helps us read chemical equations by specifying the number of moles of each substance involved. Using this information, we can determine the exact amounts of substances that are reacting.

The mass of one mole of a substance, called its molar mass, is equal to its formula mass. This value is expressed in grams per mole or g/mol. For example, the molar mass of water (H2O) is 18 g/mol, which means that one mole of water weighs 18 grams.

What is relative mass?

Scientists measure the mass of an atom by comparing it to  the mass of a neutral carbon-12 atom. We call this relative mass.

We express relative mass by referring to the unified atomic mass unit (u or Dalton). One dalton (1u) equals  the mass of a stable atom of carbon-12 or  kg.We compare all atoms to  of carbon-12 because it equals 1u.u is the unit of measurement for atomic mass. 1u equals  the mass of a carbon-12 atom.

Why do we use relative masses?

We use relative masses because atoms are so tiny that using their actual weights makes calculations tricky. Instead of using the actual mass of atoms in problems, scientists compare all atoms to a standard atom: carbon-12. They use carbon-12 because it is a stable isotope and they can measure its mass accurately. Carbon-12 is the isotope of carbon with 6 protons and 6 neutrons. They found out the carbon-12 atom weighs  grams.

Carbon-12

Scientists gave carbon-12 a mass of 12u because they found it easier to say "carbon-12 weighs 12u".  By comparing the weight of all other atoms to carbon-12 they discovered the hydrogen atom weighs  of the carbon-12 atom. So they gave hydrogen a relative mass of 1u.

hydrogen = carbon mass ÷ 12

=12u ÷ 12

=1u

What is relative atomic mass?

The atomic mass of an element will vary from one isotope to another. The figure we see for an element's atomic mass on the periodic table is the relative atomic mass.

Relative atomic mass () is the average mass of all the isotopes of an element, weighed by the abundance of each isotope on Earth.You can calculate the relative atomic mass using this formula:

What is relative molecular mass?

The weighted average of the mass of a molecule compared to  of the mass of a carbon-12 atom is called the relative molecular mass ( or RMM).

What is percentage yield?

When we compare the actual amount of product we get from a chemical reaction to the amount we theoretically could have got, it is called Percentage Yield.

Percentage yield measures the effectiveness of a chemical reaction. It tells us how much of our reactants (in percentage terms) successfully turned into a product.We calculate it like this:

Christina calculated the theoretical yield of an experiment to be 16.5g of sodium chloride. As a result of the reaction the got 12.8g of sodium chloride. Calculate the percentage yield of Christina's experiment. actual yield / theoretical yield  x 100(12.8 / 16.5)  x 100Percentage yield = 77.576 percent.

What is theoretical yield?

Theoretical yield (also known as predicted yield) is the maximum amount of product that you can get from a reaction.

Theoretical yield is the yield you would get if all the reactants in your experiment turned into a product.

Why is actual yield lower than theoretical yield?

Actual yield is the amount of product you actually get from an experiment. It is rare to get the 100 percent yield in a reaction.

Actual yield is often lower than theoretical yield because:

Some of the reactants don't convert to a product. Some of the reactants get lost in the air (if it's a gas).Impurities stop the reaction. Unwanted by-products get produced in side-reactions. The reaction reaches equilibrium.

The ideal gas equation

The Ideal Gas Law is an equation that describes the relationship between the natural properties of gases, including volume, pressure, and temperature. The equation is written as PV = nRT, where P is pressure, V is volume, n is the number of moles, R is the gas constant, and T is temperature.

To calculate the volume of 1 mole of an ideal gas at 0°C and 1 atmosphere pressure, we can use the Ideal Gas Law equation. We know that 0°C is equivalent to 273 K and 1 atm is equal to 101325 Pa. Using R = 8.31441, we can plug in these values and solve for V:

PV = nRT
101325 x V = 1 x 8.31441 x 273
V = 2269.83393101325
V = 0.0224  or 22.4

Thus, the volume of 1 mole of an ideal gas at 0°C and 1 atmosphere pressure is 22.4 L.

Before the Ideal Gas Law, scientists had discovered other relationships between temperature, pressure, and volume of gases. For example, Charles's Law states that if the amount of gas and its pressure stay the same, changing the temperature will also change the volume. Increasing the temperature will increase the volume, while lowering the temperature will decrease the volume. Boyle's Law, on the other hand, states that volume and pressure are inversely related. If the volume increases, the pressure decreases, and vice versa.

What is an ideal gas?

Ideal gases behave according to the Kinetic Gas Theory at all conditions of temperature and pressure.

That means the gas molecules have no volume and no attractive forces between each other. When you think about it, that can't be true at all! So there is no such thing as an ideal gas.

Gases that do not obey kinetic gas theory are called real gases.

Fortunately, most real gases behave in an ideal way.

What is the kinetic theory of gases?

The Kinetic Theory of Gases is an explanation of the relationships between the properties of gases. According to this theory, gases are made up of tiny particles that are in constant motion and have plenty of space between them. The theory is based on several key assumptions:

  1. Gases have molecules that move randomly in straight lines. The molecules of a gas move in a straight line until they collide with another molecule or the walls of the container.
  2. The molecules of a gas behave like rigid spheres. Even though the molecules in a gas are in constant motion, they are assumed to behave like rigid spheres that do not interact with each other except during collisions.
  3. When the molecules of gas collide with the sides of a container, it causes pressure. The pressure of a gas is caused by the constant bombardment of the container walls by the gas molecules.
  4. When the molecules collide with each other and with the container walls, they do not lose kinetic energy. The collisions between the molecules and the walls of the container are completely elastic, meaning that no energy is lost during the collision.
  5. The temperature of the gas relates to the average kinetic energy of its molecules. The average kinetic energy of the molecules in a gas is directly proportional to the temperature of the gas. This means that as the temperature of a gas increases, the average kinetic energy of its molecules also increases.

The Kinetic Theory of Gases has proven to be a useful tool for understanding the behavior of gases in a variety of settings, from atmospheric science to industrial chemistry.

The amount of substance is measured in moles (n), which is the SI unit for this quantity. One mole of a substance contains the same number of elementary entities as there are atoms in 12g of the carbon-12 isotope. This number is known as the Avogadro constant and is equal to 6.022 x10^23 entities per mole.

The unit for atomic mass is the unified atomic mass unit (u), which is defined as 1/12 of the mass of a carbon-12 atom. The relative atomic mass (Ar) of an element is the average mass of all the isotopes of that element, weighted by the abundance of each isotope on Earth.

Percentage yield is a measure of the effectiveness of a chemical reaction, expressed as the percentage of the theoretical yield that was actually obtained. The theoretical yield is the maximum amount of product that can be obtained from a reaction, while the actual yield is the amount of product obtained in an experiment.

The ideal gas law is an equation that describes the relationship between the natural properties of gases, including volume, pressure, temperature, and the number of moles. Ideal gases follow the Kinetic Gas Theory at all conditions of temperature and pressure. Real gases that do not obey this theory have intermolecular forces between their molecules and the volume occupied by the molecules is not negligible relative to the volume of the container.

The Kinetic Theory of Gases explains the behavior of gases based on the assumptions that the molecules have negligible intermolecular forces between them and the volume occupied by the molecules is negligible relative to the volume of the container. This theory has proven to be a useful tool for understanding the behavior of gases in a variety of settings, from atmospheric science to industrial chemistry.

Amount of Substance

What is a mole in chemistry?

A chemical mole is another way of saying an exact quantity. Just like we say 'a dozen' to mean 'twelve things', a mole is '602 hexillion things'. That's 602,200,000,000,000,000,000,000! We write it as 6.022 x 10^(23) for short. A mole is the SI unit for the amount of substance. The amount of elementary entities in a mole is equal to the number of atoms in 12g of the carbon-12 isotope. The number of entities per mole is the Avogadro constant or 6.022 x 10^(23).You can calculate it like this: n = m/M The mole is helpful because it allows you to read chemical equations by the number of moles of each substance. You can then work out the exact amounts of substances that are reacting.

What is Avogadro's constant?

6.022 x 10^(23) is also known as Avogadro's constant (L). It is named after an Italian scientist - Amedeo Avogadro. He discovered that equal volumes of gases when under the same conditions contain the same amount of molecules. Scientists used this discovery to calculate Avogadro's constant. Avogadro's constant (L) is the number of atoms in 12 grams of carbon-12 or 6.022 x 10^(23). We give it the units mol^-1, which you read as 'per mole'.

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