In methylamine series primary amine has more basic strength than tertiary amine; while in ethylamine series tertiary amine possess more basic strength than primary amine.Why?

hello

Usually basicity can be determined with help of following factors:

Steric factors
Electronegativity

When we talk about amines, They have irregular cases.

I am mentioning 2 of them, of methyl and ethyl substituted respectively.

For Methyl substituted

Secondary > Primary > Tertiary

For Ethyl substituted

Secondary > Tertiary > Primary

Amines Basicity is 3>2>1>NH3 in gaseous state.
But in Aqueous Media There is three factors which determine basicity 1)Inductive Effect
2)Solvation Of Ion
3) Steric Factor
For CH3 there is no Steric Factors so Basicity is of Order 2>1>3
For C2H5 and others there is steric hindrance for formation of H-Bonding So it Becomes
2>3>1.
Hopes it helps!