"Be found" vs "to be found"

a. Uranium is found in the fine sand of the Namib desert.

b. Uranium is to be found in the fine sand of the Namib desert.

c. 'In practical exam: the cation in this sample is to be found by carrying out standard analytical tests.

d. 'In practical exam: the cation in this sample is found by carrying out standard analytical tests.

Now my questions are as follows:

In the case of ''a.'' and ''b.'',

1. Are "a." and "b.'' grammatical and meaningful?

2. Do ''a.'' and ''b.'' express the general statement?

3. Do ''a.'' and ''b.'' express the same meaning and could be used interchangeably?

In the case of ''c.'' and ''d.'',

4. Are ''c.'' and ''d.'' grammatical and meaningful?

5. Do ''c.'' and ''d.'' express the general statement?

6. Do ''c.'' and ''d.'' express the same meaning and could be used interchangeabaly?

I am confused. Please explain in detail.