Here's a question I was working on:

A crystal of sulfur has a density of 1.94 g/cc.

(a) What is the mass (in grams) of a sulfur crystal with a volume of 2.534 cm3?

(b) What is the density of sulfur in kg/m3?

(c) What is the volume (in mL) of a sulfur crystal that weighs 9800 mg?

Here is the solutions I've arrived at:

(a) Well we know $\displaystyle D = M / V$ thus rearrange as $\displaystyle M = D * V$ to obtain the value of $\displaystyle 4.91596g$ which I rounded to $\displaystyle 4.92g$ as taking into account significant digits.

Correct or incorrect?

correct
(b) I know $\displaystyle D = 1.94 g/cc$ thus I can write $\displaystyle 1.94 g/cc / 1000 = 0.00194 kg/m3$

I'm not so sure about the sig figs on that final answer there, or if it's even the correct conversion at all?

corect. If your worried about sig figs switch to scientific notation.
(c) I rearrange as such: $\displaystyle V = M / D$ and plugged in $\displaystyle 9800mg / 1.94g per cc = 5051.5mL$

you're dividing mg's by g's?
Is this correct?

If you could help me with the mistakes I made and offer advice, etc. that would be appreciated. Thanks a lot for reading.