calculate the number of electrons in 2moles of boron
Answers
Answer:
Explanation:
Define Avogadro's number and explain why it is important to know.
Define the mole. Be able to calculate the number of moles in a given mass of a substance, or the mass corresponding to a given number of moles.
Define molecular weight, formula weight, and molar mass; explain how the latter differs from the first two.
Be able to find the number of atoms or molecules in a given weight of a substance.
Find the molar volume of a solid or liquid, given its density and molar mass.
Explain how the molar volume of a metallic solid can lead to an estimate of atomic diameter.
The chemical changes we observe always involve discrete numbers of atoms that rearrange themselves into new configurations. These numbers are HUGE— far too large in magnitude for us to count or even visualize, but they are still numbers, and we need to have a way to deal with them. We also need a bridge between these numbers, which we are unable to measure directly, and the weights of substances, which we do measure and observe. The mole concept provides this bridge, and is central to all of quantitative chemistry.
Counting Atoms: Avogadro's Number
Owing to their tiny size, atoms and molecules cannot be counted by direct observation. But much as we do when "counting" beans in a jar, we can estimate the number of particles in a sample of an element or compound if we have some idea of the volume occupied by each particle and the volume of the container. Once this has been done, we know the number of formula units (to use the most general term for any combination of atoms we wish to define) in any arbitrary weight of the substance. The number will of course depend both on the formula of the substance and on the weight of the sample. However, if we consider a weight of substance that is the same as its formula (molecular) weight expressed in grams, we have only one number to know: Avogadro's number.
Avogadro's number
Avogadro's number is known to ten significant digits:
NA=6.022141527×1023.(1.4.1)
However, you only need to know it to three significant figures:
NA≈6.02×1023.(1.4.2)
So 6.02×1023 of what? Well, of anything you like: apples, stars in the sky, burritos. However, the only practical use for NA is to have a more convenient way of expressing the huge numbers of the tiny particles such as atoms or molecules that we deal with in chemistry. Avogadro's number is a collective number, just like a dozen. Students can think of 6.02×1023 as the "chemist's dozen".
Before getting into the use of Avogadro's number in problems, take a moment to convince yourself of the reasoning embodied in the following examples.
Example 1.4.1 : Mass ratio from atomic weights
The atomic weights of oxygen and carbon are 16.0 and 12.0 atomic mass units ( u ), respectively. How much heavier is the oxygen atom in relation to carbon?
Solution
Atomic weights represent the relative masses of different kinds of atoms. This means that the atom of oxygen has a mass that is
16u12u=43≈1.33
as great as the mass of a carbon atom.