I'm trying to write my lab report up and I'm getting really confused about a certain calculation!

I made a 1% agarose gel, and Basically, I added 5 μL of ethidium bromide to a solution (solution= 0.5 g agarose and 50 μL TAE) and the bottle said the ethidium bromide had a concentration of 5 μg/μL. so what concentration of ethidium bromide am I adding to my solution?

Help would be much appreciated!

5 μg/μL of ethidium bromide in 50 μL TAE

so in 100μL ??

by calculation: 100x5/50=10 μL

You do not add a concentration of EtBr, but you add a certain amount into a volume resulting in a given concentration.

 

Your stock of EtBr has 5µg/µl. You took out 5µl which is a total of 5µg. Then divide that amount by your final volume to get the concentration.

ok. I understand half of that. But why are you taking out 5 μg? I dont understand how you get to that?

Final amount = 5 µL × 5 µg/µL = 25 µg.

Final volume = 5 µL + 50 µL = 55 µL.

Final concentration = 25 µg / 55 µL = 0.455 µg/µL.

 

But you asked what concentration of ethidium bromide you were adding to the solution, and the answer was already written on the bottle: 5 μg/μL.

My bad. It should have been 25 µg, not 5.