The initial concentration of a reactant in a first order reaction is 0.620 molar. What will be its concentration after 3 half-lives?
I did 0.620 -> 0.31 ->0.155
0.155M will be the concentration. Is this correct?
You're ok as far as you went but you only went through 2 half lives.
0.620 start becomes 0.310 after 1.
0.310 becomes 0.155 after 2.
0.155 becomes ..... after 3.
Oh ok I did realize that. Now I got 0.0775M
Yes, your calculation is correct. In a first-order reaction, the concentration of a reactant decreases by half after each half-life. Therefore, after 3 half-lives, the concentration of the reactant will be 0.155 Molar.
Yes, your approach to calculating the concentration after 3 half-lives is correct. In a first-order reaction, the concentration of a reactant decreases by half during each half-life.
To calculate the concentration after each half-life, you can follow these steps:
1. Calculate the first half-life: In order to determine the half-life of a first-order reaction, you need to know the rate constant (k). The half-life (t1/2) can be calculated using the equation: t1/2 = ln(2) / k. If the rate constant is not given, you won't be able to calculate the exact half-life.
2. Calculate the concentration after each half-life: Start with the initial concentration (0.620 M). After the first half-life, the concentration will be halved, resulting in a concentration of 0.310 M. After the second half-life, the concentration will again be halved, resulting in a concentration of 0.155 M. Finally, after the third half-life, the concentration will be halved once more, resulting in a concentration of 0.078 M.
Therefore, the correct concentration after 3 half-lives is 0.078 M, not 0.155 M as you mentioned.