Molality is defined as the number of moles of solute in 1 kg of solvent.
molality of solution to be prepared is 0.50 molal
this means that in 1000 g of water there should be 0.50 mol of NaCl
if 1000 g of water should contain - 0.50 mol
then 750.0 g of water requires - 0.50 mol/kg x 0.750 kg = 0.375 mol
mass of NaCl in 0.375 mol - 58.5 g/mol x 0.375 mol = 21.9 g
therefore a mass of 21.9 g of NaCl is required
For each compound, i.e. galena, cerussite and anglesite calculate the quantity of Pb contained.
For that, you need to use the molecular formula and the atomic weights of the elements.
Galena, PbS
Atomic weights
Pb: 207 uma
S: 32 uma
Molar mass = 207 + 32 = 239 g/mol
Proportion of Pb in the compound = 207/239
% of Pb from galena: 38.0% * [207/239]
Cerussite
Pb CO3
Atomic weights
Pb: 207
C: 12
O: 16
Molar mass = 207 + 12 + 3*16 = 267 g/mol
% of Pb coming from the cerussite
25.0% * [207/267]
Anglesite, PbSO4
Molar mass: 207 + 32 + 4*16 = 303 g/mol
% of Pb coming from anglesite = 17.4% * [207/303]
Total quantity of Pb from the rock (call X the quantity of rock)
[38.0%*207/239 + 25.0%*207/267 + 17.4%*207/303]*X.
And that must be equal to 5.8 metric tons = 5800 kg
The result of the calculations are:
[32.91% + 19.38% + 11.89%]X = 5800 KG
64.18% X = 5800 KG
0.6418 X = 5800 KG
X = 5800 / 0.6418
X = 9037 KG
Which you have to express with two significan figures, i.e. 9000 kg
Gravity causes mass wasting.
I think it's A, I hope this helps