When solving a problem for dissolving a salt in water, how do you know when part of the compound is negligible? As an example:
"A 15.5 g sample of CH3NH3Cl is dissolved in water to make 450. mL solution (Kb = 3.6 x 10^-4). What is the pH of the solution?"
How do you know to set it up as a deprotonation equation and what happens to the Cl^-?
Re: The pH of salt solutions
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Re: The pH of salt solutions
Essentially, what you want to do is focus on the separate components of your salt. In other words, separate the cation and the anion.
CH3NH3Cl --> CH3NH3+ + Cl-
Remember, salts are the products of neutralization reactions (acid+base). Therefore, the cations and anions of salts must be derived from either an acid or a base. Meaning, the cations and anions of salts are CONJUGATES of acids and bases.
CH3NH3+ has an extra positive charge --> remove an H+ --> CH3NH3+ is the conjugate acid to the base CH3NH2
Cl- has a negative charge --> add an H+ --> Cl- is the conjugate base to the acid HCl
Now, we need to look at the strength of the conjugates. Since CH3NH2 has a nitrogen with a lone pair, you know this is a weak base. Since HCl is an inorganic acid (and one of the strong acids you should just memorize), you know this is a strong acid.
Combine what we know:
CH3NH3+ is the conjugate acid to a WEAK base
Cl- is the conjugate base to a STRONG acid
Since strong acids are known to disassociate completely, we treat their reactions as though they are irreversible (although, in reality, they are actually reversible). The reaction HCl --> H+ + Cl- is treated as though it is one way, so you can assume from this that when Cl- is disassociated from the salt, it will not react with the water in the solution (in this case, water would be your acid). Therefore, the Cl- becomes a spectator ion and you can exclude it from the reaction.
Weak bases, on the other hand, although they do still ionize, they don't ionize completely. Their BH+ (conjugate acids like CH3NH3+) are still partially reacting with the OH- product to go back and forth between base and conjugate acid. So, BH+ is still constantly reacting with the water and must be kept within the equation.
Your final chemical equation after eliminating spectator ions (Cl-) would be:
CH3NH3+ H2O--> CN3NH2 + H3O+
Since CH3NH3+ is the conjugate acid (and you can see H3O+ being produced in the chemical equation above), your salt is acidic.
CH3NH3Cl --> CH3NH3+ + Cl-
Remember, salts are the products of neutralization reactions (acid+base). Therefore, the cations and anions of salts must be derived from either an acid or a base. Meaning, the cations and anions of salts are CONJUGATES of acids and bases.
CH3NH3+ has an extra positive charge --> remove an H+ --> CH3NH3+ is the conjugate acid to the base CH3NH2
Cl- has a negative charge --> add an H+ --> Cl- is the conjugate base to the acid HCl
Now, we need to look at the strength of the conjugates. Since CH3NH2 has a nitrogen with a lone pair, you know this is a weak base. Since HCl is an inorganic acid (and one of the strong acids you should just memorize), you know this is a strong acid.
Combine what we know:
CH3NH3+ is the conjugate acid to a WEAK base
Cl- is the conjugate base to a STRONG acid
Since strong acids are known to disassociate completely, we treat their reactions as though they are irreversible (although, in reality, they are actually reversible). The reaction HCl --> H+ + Cl- is treated as though it is one way, so you can assume from this that when Cl- is disassociated from the salt, it will not react with the water in the solution (in this case, water would be your acid). Therefore, the Cl- becomes a spectator ion and you can exclude it from the reaction.
Weak bases, on the other hand, although they do still ionize, they don't ionize completely. Their BH+ (conjugate acids like CH3NH3+) are still partially reacting with the OH- product to go back and forth between base and conjugate acid. So, BH+ is still constantly reacting with the water and must be kept within the equation.
Your final chemical equation after eliminating spectator ions (Cl-) would be:
CH3NH3+ H2O--> CN3NH2 + H3O+
Since CH3NH3+ is the conjugate acid (and you can see H3O+ being produced in the chemical equation above), your salt is acidic.
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