So, if you have a sample with this number of lithium atoms, that sample is going to have a mass of 6. Whatever its average atomic mass is in terms of unified atomic mass units, if you have that number of the atom, you will have a mass of that same number in terms of grams. Now, you might be saying, is there a name for this number, and there is indeed a name, and it is called Avogadro's number, named in honor of the early 19th century Italian chemist, Amedeo Avogadro.
And, in most contexts, because you're not normally dealing with data with this many significant digits, we will usually approximate it as 6. Now, there's another word that it's very useful to familiarize yourself with in chemistry, and that's the idea of a mole. Now, what is a mole? It is not a little mark on your cheek. It is not a burrowing animal. Actually, it is both of those things, but, in a chemistry context, a mole is just saying you have this much of something.
The word mole was first used by the German chemist Wilhelm Ostwald at the end of the 19th century, and he came up with the word because of its relation to molecule. Now, what does that mean? Well, think about the word dozen. If I say I've got a dozen of eggs, how many eggs do I have? Well, if I have a dozen of eggs, that means I have 12 eggs. So, if I say I have a mole of lithium atoms, how many lithium atoms do I have?
That means that I have 6. Exact same idea, it's just that Avogadro's number is much hairier of a number than a dozen. So, let's use our new found powers of the mole and Avogadro's number to start doing some useful things. Let's say that someone were to walk up to you and say, hey, you, I have a How many atoms of germanium am I dealing with?
Calcium nitrate, Ca NO 3 2 , is used as a component in fertilizer Determine the molar mass of calcium nitrate.
Step 1: List the known and unknown quantities and plan the problem. First we need to analyze the formula. Since the Ca lacks a subscript, there is one Ca atom per formula unit. The 2 outside the parentheses means that there are two nitrate ions per formula unit and each nitrate ion consists of one nitrogen atom and three oxygen atoms.
Thus, 1 mol of calcium nitrate contains 1 mol of Ca atoms, 2 mol of N atoms, and 6 mol of O atoms. Use the molar masses of each atom together with the number of atoms in the formula and add together. Read the material at the link below and work the problems at the end:.
How can we get more product? Chemical manufacturing plants are always seeking to improve their processes. One of the ways this improvement comes about is through measuring the amount of material produced in a reaction. By knowing how much is made, the scientists and engineers can try different ways of getting more product at less cost. The molar mass of any substance is the mass in grams of one mole of representative particles of that substance. The representative particles can be atoms, molecules, or formula units of ionic compounds.
This relationship is frequently used in the laboratory. Suppose that for a certain experiment you need 3. Since calcium chloride is a solid, it would be convenient to use a balance to measure the mass that is needed. The molar mass of CaCl 2 is Dimensional analysis will allow you to calculate the mass of CaCl 2 that you should measure.
When you measure the mass of g of CaCl 2 , you are measuring 3. Calcium chloride is used as a drying agent and as a road deicer. Chromium metal is used for decorative electroplating of car bumpers and other surfaces. Find the mass of 0. One conversion factor will allow us to convert from the moles of Cr to mass. Since the desired amount was slightly more than one half of a mole, the mass should be slightly more than one half of the molar mass.
The answer has three significant figures because of the 0. A similar conversion factor utilizing molar mass can be used to convert from the mass of an substance to moles. In a laboratory situation, you may perform a reaction and produce a certain amount of a product which can be massed. It will often then be necessary to determine the number of moles of the product that was formed. The next problem illustrates this situation.
A certain reaction produces 2. Determine the number of moles produced in the reaction. One conversion factor will allow us to convert from mass to moles. The molar mass is The relatively small mass of product formed results in a small number of moles. Read the material in the link below and work the problems at the end. How much gas is there? Avogadro was interested in studying gases. He theorized that equal volumes of gases under the same conditions contained the same number of particles.
Other researchers studied how many gas particles were in a specific volume of gas. Eventually, scientists were able to develop the relationship between number of particles and mass using the idea of moles. Now you have seen how to convert back and forth between moles and mass of a substance in grams. We can combine the two types of problems into one. Mass and number of particles are both related to grams.
In order to convert from mass to number of particles or vice-versa, it will first require a conversion to moles. Conversion from number of particles to mass or from mass to number of particles requires two steps. How many molecules is Use two conversion factors. The first converts grams of Cl 2 to moles. The second converts moles of Cl 2 to the number of molecules. The problem is done using two consecutive conversion factors. There is no need to explicitly calculate the moles of Cl 2.
How do scuba divers know if they will run out of gas? Knowing how much gas is available for a dive is crucial to the survival of the diver. A basic knowledge of gas behavior allows the diver to assess how long to stay under water without developing problems. Volume is a third way to measure the amount of matter, after item count and mass. With liquids and solids, volume varies greatly depending on the density of the substance.
This is because solid and liquid particles are packed close together with very little space in between the particles. However, gases are largely composed of empty space between the actual gas particles see Figure below. Gas particles are very small compared to the large amounts of empty space between them.
In , Amedeo Avogadro explained that the volumes of all gases can be easily determined. Since the total volume that a gas occupies is made up primarily of the empty space between the particles, the actual size of the particles themselves is nearly negligible.
A given volume of a gas with small light particles such as hydrogen H 2 contains the same number of particles as the same volume of a heavy gas with large particles such as sulfur hexafluoride, SF 6. Gases are compressible, meaning that when put under high pressure, the particles are forced closer to one another. This decreases the amount of empty space and reduces the volume of the gas.
Gas volume is also affected by temperature. When a gas is heated, its molecules move faster and the gas expands. Because of the variation in gas volume due to pressure and temperature changes, the comparison of gas volumes must be done at one standard temperature and pressure.
The molar volume of a gas is the volume of one mole of a gas at STP. At STP, one mole 6. A mole of any gas occupies The Figure below illustrates how molar volume can be seen when comparing different gases.
Each contains 1 mole or 6. However, the mass of each gas is different and corresponds to the molar mass of that gas: 4. At standard temperature and pressure, 1 mole of any gas occupies How can you tell how much gas is in these containers?
Small gas tanks are often used to supply gases for chemistry reactions. A gas gauge will give some information about how much is in the tank, but quantitative estimates are needed so the reaction will be able to proceed to completion. Knowing how to calculate needed parameters for gases is very helpful to avoid running out too early. Molar volume at STP can be used to convert from moles to gas volume and from gas volume to moles. Many metals react with acids to produce hydrogen gas.
A certain reaction produces How many moles of hydrogen were produced? Apply a conversion factor to convert from liters to moles. The volume of gas produced is nearly four times larger than the molar volume. The fact that the gas is hydrogen plays no role in the calculation. What volume does 4. References Keenan, Charles W. General College Chemistry.
New York: Haper and Row, Mortimer, Charles E. Chemistry a Conceptual Approach. New York: Van Nostrand Reinhold, Jones, Loretta and Atkins, Peter. Chemistry: Molecules, Matter, and Change. New York: W. Freeman, Petrucci, Ralph H. General Chemistry: Principles and Modern Applications. These relationships can be used to convert between units. What is the molar mass of NaOH? If the equation is arranged correctly, the mass units g cancel out and leave moles as the unit.
According to the periodic table, the atomic mass of nickel Ni is Therefore, we can divide Privacy Policy. Skip to main content. Mass Relationships and Chemical Equations. Search for:. Key Takeaways Key Points The mole allows scientists to calculate the number of elementary entities usually atoms or molecules in a certain mass of a given substance.
This can also be written as 6. Key Terms mole : The amount of substance of a system that contains as many elementary entities as there are atoms in 12 g of carbon Molar Mass of Compounds The molar mass of a particular substance is the mass of one mole of that substance.
Learning Objectives Calculate the molar mass of an element or compound.
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