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# Chemistry concentration question watch

1. Q4.Use the information below to answer this question.
A saturated solution of magnesium hydroxide, Mg(OH)2, contains 0.1166 g of Mg(OH)2 in 10.00 dm3 of solution. In this solution the magnesium hydroxide is fully dissociated into ions.
Which one of the following is the concentration of Mg2+(aq) ions in the saturated solution?
A 2.82 × 10^−2 mol dm−3
B 2.00 × 10^−3 mol dm−3
C 2.82 × 10^−3 mol dm−3
D 2.00 × 10^−4 mol dm−3

I've attempted this but I've got a different answer to the mark scheme. I did 0.1166/58.3=0.002 which is apparently wrong. could someone help?
2. Is it D) 2.00x10^-4

grams=Mr x moles -> 0.1166/58.3=0.002

but you want concentration so you then use n=cv -> rearranged to c=n/v -> 0.002/10 = 2x10^-4 mol dm^3

You worked out just the number of moles, not the concentration
3. (Original post by ScienceWise2017)
Is it D) 2.00x10^-4

grams=Mr x moles -> 0.1166/58.3=0.002

but you want concentration so you then use n=cv -> rearranged to c=n/v -> 0.002/10 = 2x10^-4 mol dm^3

You worked out just the number of moles, not the concentration
oh of course, i didnt read the question properly as usual. thank you!

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