chemistry help?

Question

plz answer the following questions and show working. thanks for your help in advance!

a) what is the gram formula mass of Mg(NO3)2 (show working)
b) how many moles in 16g CuO
c) what concentration of solutions is obtained if 4.0g of NaOH is dissolved in water and made up 250cm^3 of solution

Answer 1

i really think you should be doing your own homework.

Answer 2

a).
The 'gram formula mass' is an archaic (old fashioned) phrase for molar mass.
Mg x 1 = 24 x 1 = 24 24 is Mr(Mg) a '1' is the no. of atoms in the molecule.
N x 2 = 14 x 2 = 28
O x 6 = 16 x 6 = 96
24 + 28 + 96 = 148 = Mr(Mg(NO3)2
moles = mass(g)/Mr
1 mole = mass(g)/148
mass/g = 1 x 148 = 148 g ( the molar mass - gram formula mass).

b).
Cu x 1 = 64 x 1 = 64
O x 1 = 16 x 1 = 16
64 + 16 = 80 (Mr(CuO))
moles(CuO) = 16g/80 = 0.2 moles

c).
Na x 1 = 23 x 1 =23
O x 1 = 16 x 1 = 16
H x 1 = 1 x1 = 1
23 + 16 + 1 = 40 (Mr(NaOH))
Moles(NaOH) = 4.0g/40 = 0.1moles
NB 250cm^3 = 1/4 litre
So :-
0.1 mol(NaOH) is dissolved in 0.25 litres.
Multiply by '4' to bring to 1 litre.
0.4 mol(NaOH) is dissolved in 1 litre.
Sometimes written as 0.4M (NaOH).
So concentration is 0.4 mol dm^-3

Answer 3

These take practise, and you really have to be able to do them yourself. And you'll need a periodic table

Moles = Mass /Molecular mass

Moles = Volume x Concentration

if the volume is in cm cubed, then do ...

moles = (volume x concentration) / 1000

Answer 4

ok, so im not directly answering your Qs, but this website might help: http://www.gcsescience.com/m.htm (i use this site when im stuck on chemistry)