Dilutions

A student prepared a 15 mL diluted HCl solution from 5 mL of 5 M stock HCl. What is the molarity of the prepared solution?

4 mL of stock solution was added to 10 mL of buffer to form a final 2 M diluted solution. What was the molarity of the original stock solution?

100 mL of 0.5 M NaOH was produced from a 4 M stock NaOH solution. How much of the stock solution was used to create the final solution?

20 mL of 3 M NaCl was added to buffer to produce a 50 mL solution. Then 5 mL of the new solution was added to buffer to produce a 100 mL solution. What is the NaCl concentration of the new solution?

Scientist prepared a diluted solution of lactase, an enzyme that breaks down lactose, by first taking 2 mL of purified lactase and adding it to 8 mL of buffer. Following this they took 2 mL of the new solution and added enough buffer to form a 100 mL solution. If [Lactase] equals the original concentration of the purified sample what is the final concentration of the diluted lactase solution?

Light microscopy can be used to determine the number of bacterial colonies in a culture. However, if the number of colonies goes above a certain threshold it is impossible to count them using this technique. To get around this problem a sample from the original plate can be diluted down through a series of dilutions and re-plated to lower the number of colonies to a countable level. If a plate contains 40 colonies per field after undergoing a 3:1 dilution followed by a 40:3 dilution then the number of colonies per field on the original plate is: