Avogadro Constant

What mass of magnesium chloride (MgCl2) must be taken if it is to contain 10.65g of chloride ions?

i'm very confused, could you please talk me through the steps, many thanks

Reply 1

cpchem

14

OK... here goes...

10.65g of chloride ions (Cl^-) are required. How many moles is this?
Divide the mass by the molar mass of chloride ions (35.5 g mol^-1)

10.65g / 35.5 g mol^-1 = 0.3 mol

The ratio of chloride ions to MgCl₂ is 2:1, so 0.15 mol of MgCl₂ are required.

Multiply this figure by the molar mass of magnesium chloride (95 g mol^-1) to get the final answer: you need 14.25 g of MgCl₂

Reply 2

Stompy55

1

How do you know the ratio?

Reply 3

zzzzzzDavid

1

1) Find the percentage by mass of the chloride ions in the magnesium chloride.The percentage by mass of the chloride ions = (the relative formula mass of two chloride anions/the total relative formula mass)*100% = (35.5*2)/(24 35.5*2) * 100%= 74.74%∵ 74.74% of the magnesium chloride is made up of chloride anions∴ 74.74% * the mass of magnesium chloride = the mass of chloride anions∴ 74.74% * the mass of magnesium chloride = 10.65 g∴ The mass of magnesium chloride = 10.65 g/74.74% = 14.25 g——————————————————————————————There are two chloride anions in every MgCl2, thus the ratio of chloride ions to MgCl2*is 2:1Since there are 0.3mol of chloride anionsThere must be 0.15mol of MgCl2Convert the molar mass of MgCl2 to grams r.f.m is the number of grams per mol. To find out how many grams of MgCl2, we use multiply the mole by the grams per mole to get grams. r.f.m (g/mol), mass(g), mole(mol)Mass = r.f.m*mol0.15mol*r.f.m of MgCl20.15mol*95 Answer: 14.25 grams

Reply 4

zzzzzzDavid

1

1) Find the percentage by mass of the chloride ions in the magnesium chloride.

The percentage by mass of the chloride ions = (the relative formula mass of two chloride anions/the total relative formula mass)*100%
= (35.5*2)/(24+35.5*2) * 100%
= 74.74%

∵ 74.74% of the magnesium chloride is made up of chloride anions

∴ 74.74% * the mass of magnesium chloride = the mass of chloride anions
∴ 74.74% * the mass of magnesium chloride = 10.65 g
∴ The mass of magnesium chloride = 10.65 g/74.74%
= 14.25 g

——————————————————————————————
There are two chloride anions in every MgCl2, thus the ratio of chloride ions to MgCl2 is 2:1

Since there are 0.3mol of chloride anions
There must be 0.15mol of MgCl2

Convert the molar mass of MgCl2 to grams
r.f.m is the number of grams per mol. To find out how many grams of MgCl2, we use multiply the mole by the grams per mole to get grams.

r.f.m (g/mol), mass(g), mole(mol)
Mass = r.f.m*mol

0.15mol*r.f.m of MgCl2

0.15mol*95

14.25 grams

Reply 5

samuel3000

3

Original post by cpchem

OK... here goes...

10.65g of chloride ions (Cl^-) are required. How many moles is this?
Divide the mass by the molar mass of chloride ions (35.5 g mol^-1)

10.65g / 35.5 g mol^-1 = 0.3 mol

The ratio of chloride ions to MgCl₂ is 2:1, so 0.15 mol of MgCl₂ are required.

Multiply this figure by the molar mass of magnesium chloride (95 g mol^-1) to get the final answer: you need 14.25 g of MgCl₂

Thank you!
This really helped

Avogadro Constant

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