What is the molarity of a solution that contains 28g of koh in 2l of solution?

You should know hte formula c=n/v (c is the molarity, n is moles and v is volume)

To change from mass to moles we use n=m/M (n is moles, m is mass and M is molar mass)

We can find the M of KOH from the periodic table (K=39g/mol, O=16g/mol, H=1g/mol)
Therefore, M of KOH is 39+16+1=56g/mol

Now lets find n: n=28/56=0.5mol

Finally, the molarity is c=0.5/2=0.25M

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.