Measuring pH Flashcards
[x] means
the concentration of x
what is the equation to show when water molecules undergo self ionisation
H2O(l) + H2O(l) ⇌ H3O+(aq) + OH−(aq)
how does water behave when reacting with itself
as both a very weak base and very weak acid
Kw
ionic product of water
formula to calculate Kw
[H3O+][OH-]=1x10^-14 at 25°C
how is the Kw formula justified
all aqeuous solutions conatin H3O+ ions and OH- ions and the product of their molar concentrations is always 1x10^-14 at 25°C and therfore if either [H3O+][OH-]increases than the concentration of the other must decrease proportionatley
formula to calculate concentration of H3O+ from known concentration of OH-
[H3O+] = 1x10^-14/[OH-]
formula to calculate concentration of OH- from known concentration of H3O+
[OH-} = 1x10^-14/[H3O+]
[H3O+][OH-] is known as
the ionic product of water
in pure water and neutral solutions [H3O+]
10^-7
in pure water and neutral solutions [OH-]
10^-7
in acidic solutions [H3O+]
[H3O+]>10^-7
in acidic solutions [OH-]
[OH-}<10^-7
in basic solutions [H3O+]
[H3O+]<10^-7
in basic solutions [OH-]
[OH-}>10^-7
how to calculate [H3O+] from known acid concentration
if the acid contains 1 hydrogen atom, its [H3O+] is equal to its concentration,if the acid contains 2 hydrogen atoms its [H3O+] is double its concentration
how to calculate [OH-] from base
if the base contains 1 hydrogen atom, its [OH-] is equal to its concentration,if the base contains 2 hydrogen atoms its [OH-] is double its concentration
formula to calculate pH
pH=-log10[H3O+]
at 25°C, neutral solutions have a pH of
7
at 25°C, acidic solutions have a pH of
less than 7
at 25°C, basic solutions have a pH of
more than 7
a solution of pH 2 has how many more H3O+ ions than a pH of 3
ten times the concentration of the H3O+
how to calculate the [H3O+] from known pH
[H3O+] = 10^-pH
pH metres do what
provide a specific quantitative measurement of pH
how do pH metres work
by measuring the difference in electrical potential between two probes due to the concentration of charged particles or ions in the solution
what are indicators
substances that change colour when they are added to an acidic or basic solution
what is universal indicator
a mixture of a variety of other indicators that can be used to measure the approximate pH of a solution
litmus paperr
one of tthe simplest and most common indicatorrs which uses a dye derived from lichens
litmus paper in acidic solutions
blue litmus paper will turn red
littmus paper in basic solutions
red litmus paper will turn blue
why as indicators less accurate then pH metres
indicators only provide a pH range for a given solution
accuracy of pH metres
highly accurate, usually correct to +-0.01 pH uints and is built into the calibration of the metre
why are pH metres more precise
because there is no change to the calibration of the instrument between measurements
why are indicators less precise
the estimated pH of a given solution may differ greatly between multiple trials
end point
the point at which the indicator changes colour
