# Thread: rate problem with separation of variables

1. ## rate problem with separation of variables

The rate at which a substance dissolves in a solution is proportional to the amount of the substance that is present. Thirty grams of sodium chloride are put in a litre of water , and is found that 1 hr later only 10g are left. How much will be left after 30min?

i have no idea how to do this!

2. Originally Posted by shawli
The rate at which a substance dissolves in a solution is proportional to the amount of the substance that is present. Thirty grams of sodium chloride are put in a litre of water , and is found that 1 hr later only 10g are left. How much will be left after 30min?

i have no idea how to do this!
The rate of change of the substance is porportional to the amount of the substance. The instantanous rate of change in the derivitive. $N'=\alphaN$Use this to work out what the feunction for N is. Then input your numbers.

3. Originally Posted by shawli
The rate at which a substance dissolves in a solution is proportional to the amount of the substance that is present. Thirty grams of sodium chloride are put in a litre of water , and is found that 1 hr later only 10g are left. How much will be left after 30min?

i have no idea how to do this!
The rate of change of the substance is proportional to the amount of the substance. The instantaneous rate of change in the derivative. $N' =\alpha N$Use this to work out what the function for N is. Then input your numbers.