The mole and the Avogadro constant Notes for iGCSE Chemistry Notes CIE prepared by iGCSE Teachers
The mole and the Avogadro constant Notes for iGCSE
Core Syllabus
- State that concentration can be measured in g /dm3 or mol/dm3
Supplement Syllabus
State that the mole, mol, is the unit of amount of substance and that one mole contains $6.02 \times 10^{23}$ particles, e.g. atoms, ions, molecules; this number is the Avogadro constant Use the relationship amount of substance $(\mathrm{mol})=\frac{\text { mass }(\mathrm{g})}{\text { molar mass }(\mathrm{g} / \mathrm{mol})}$ to calculate:
(a) amount of substance
(b) mass
(c) molar mass
(d) relative atomic mass or relative molecular/formula mass
(e) number of particles, using the value of the Avogadro constantUse the molar gas volume, taken as $24 \mathrm{dm}^3$ at room temperature and pressure, r.t.p., in calculations involving gases
Calculate stoichiometric reacting masses, limiting reactants, volumes of gases at r.t.p., volumes of solutions and concentrations of solutions expressed in $\mathrm{g} / \mathrm{dm}^3$ and $\mathrm{mol} / \mathrm{dm}^3$, including conversion between $\mathrm{cm}^3$ and $\mathrm{dm}^3$
Use experimental data from a titration to calculate the moles of solute, or the concentration or volume of a solution
Calculate empirical formulae and molecular formulae, given appropriate data
Calculate percentage yield, percentage composition by mass and percentage purity, given appropriate data
GEOMETRIC SEQUENCE….
- THE DEFINATION..
I give you the first term of a sequence, say \(u_1\) =5 and I ask you to multiply by a fixed number, say r =2, in order to find the next term. The following sequence is generated:
5, 10, 20, 40, 80, …
Such a sequence is called geometric. That is, in a geometric sequence the ratio between any two consecutive terms is constant.
In other words , we can say , geometric sequence is a sequence where there is a common ratio between consecutive number.
For determining geometric sequence , we only need
- The first term
- The common ratio
- Common ratio: The common ratio is the constant factor by which each term in a geometric sequence is multiplied to get the next term.
EXAMPLE 1 (b) \(u_1\) =5, r = 10 the sequence is 5, 50, 500, 5000, … (c) \(u_1\) =1, r = -2 the sequence is 1,-2, 4,-8, 16, … (d) \(u_1\) =1, r = \(\frac{1}{2}\) the sequence is 1, \(\frac{1}{2}\), \(\frac{1}{4}\), \(\frac{1}{16}\), \(\frac{1}{32}\), ….. (e) \(u_1\) =1, r = \(\frac{-1}{2}\) the sequence is 1, \(\frac{-1}{2}\), \(\frac{1}{4}\), \(\frac{1}{16}\), \(\frac{-1}{32}\), ….. |
ATTENTION!!
- The common ratio r may also be negative! In this case the signs alternate (+, -, +, -, …) [see (c) and (e) above].
- The common ratio r may be between -1 and 1, that is |r|<1. In such a sequence the terms approach 0 [see (d) and (e) above]
- THE \(N^{th}\) TERM FORMULA
The nth term formula for a geometric sequence is \(u_n =u_1r^{n-1}\)
where,
- \(u_1\) is the first term
- n is the term number
- r is the common ratio
Indeed, let us think:
In order to find \(u_5\) , we start from u1 and then multiply 4 times by the ratio r
Hence, \(u_5 = u_1r^4\)
Similarly, \(u_{10} = u_1r^9\), \(u_{50} = u_1r^{49}\) and so on…..
EXAMPLE 2 (b) the 100th term Solution \(u_{100} = u_1r^{99} = 3.2^{99}\) EXAMPLE 3 Solution \(u_{10}=u_1r^9 = 196830 = 10 . r^9\) \(r^9= 19683\) \(r= \sqrt[9]{19683} = 3\) Therefore , \(u_3 = u_1r^2 = 10.3^2 = 90\) |
REMEMBER!!
Our first task in a G.S. is to find the basic elements, \(u_1\) and r , and then anything else.
- SUM OF N TERMS(\(S_n\)
Given that r ≠ 1 , the result is given by
\(S_n = \frac{u_1(r^n-1)}{r-1}\) or \(S_n = \frac{u_1(1-r^n)}{1-r}\)