Final answer:
After performing four 1:5 dilutions on a 10 ml solution with an initial concentration of 40g/ml, the concentration of tube 4 is 0.064g/ml.
Step-by-step explanation:
To perform a 1:5 dilution four times on a 10 ml solution with an initial concentration of 40g/ml, we repeatedly dilute the solution by adding it to four times the volume of solvent. For the first dilution, we take 1 ml of the initial solution and add 4 ml of solvent, which results in a 5 ml solution. Since the volume is now five times greater, the new concentration is one-fifth of the initial, so 40g/ml divided by 5 equals 8g/ml.
For the second dilution, again, 1 ml of the now 8g/ml solution is taken and mixed with 4 ml of solvent. This dilution process gives us a concentration of 8g/ml divided by 5, which equals 1.6g/ml. We repeat this process two more times: the third dilution yields a concentration of 1.6g/ml divided by 5, which equals 0.32g/ml, and the fourth dilution results in a concentration of 0.32g/ml divided by 5, which equals 0.064g/ml.
Therefore, the concentration of tube 4 after four 1:5 dilutions is 0.064g/ml.