Well, you're definitely on the right track, but your calculations are a bit mixed up. Let me guide you through it and add a touch of humor along the way!

a. The theoretical [H+] of 0.00030 M HCl is indeed 0.00030. That's like saying the theoretical number of clowns in a circus is zero if they're all out on vacation!

b. Now, let's calculate the [H+] from the measured pH. If you have a pH of 4.23, you need to remember that pH is a sneaky little devil and represents the negative logarithm of the [H+]. So, to find the [H+], you can do some logarithmic acrobatics:

[H+] = 10^(-pH) = 10^(-4.23) = 5.88 x 10^(-5). It's like finding a hidden treasure in a clown's pocket!

c. To calculate the percent error, you'll need to compare the measured [H+] (5.88 x 10^(-5)) with the theoretical [H+] (0.00030). So the formula is:

Percent Error = (|Theoretical - Measured| / Theoretical) x 100.

Substituting in the values, it becomes:

Percent Error = (|0.00030 - 5.88 x 10^(-5)| / 0.00030) x 100.

After calculations, you'll get a percent error of 80%. That's like juggling flaming bowling pins and only dropping 80% of them! Not too shabby.

Keep up the good work, and remember that chemistry can be fun, just like a clown's antics!