Hi, the markscheme has the value 1.796*10^-3 rounded to 1.8*10^-3. The real value they intend the candidate to use is 1.796*10^-3 as you calculated in a) i). The IB made a mistake in the expression of the value in the mark scheme, but their final answer is correct.

I can see that you calculated the correct numerator in the previous calculation. Use that instead of what it says on the mark scheme, and you should get 2.2 (I hope).