Question

The amount ofcalcium present in milk can be determined by adding oxalate to asample and measuring the massof calcium oxalate precipitated. What is the mass percent ofcalcium if 0.429 g of calcium oxalate forms in a125-g sample of milk when excess aqueous sodium oxalate isadded? Na2C2O4(aq) +Ca2+(aq) → CaC2O4(s) +2Na+(aq) a. 0.107% b. 0.202% c. 0.343% d. 1.10% e.1.37%

Answer 1

Answer: The mass percent of calcium in milk is 0.107 %

Step-by-step explanation:

To calculate the number of moles, we use the equation:

`\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}` .....(1)

Given mass of calcium oxalate = 0.429 g

Molar mass of calcium oxalate = 128.1 g/mol

Putting values in equation 1, we get:

`\text{Moles of calcium oxalate}=(0.429g)/(128.1g/mol)=0.0033mol`

The given chemical equation follows:

`Na_2C_2O_4(aq.)+Ca^(2+)(aq.)\rightarrow CaC_2O_4(s)+2Na^+(aq.)`

Sodium oxalate is present in excess. So, it is considered as an excess reagent. And, calcium ion is a limiting reagent because it limits the formation of product.

By Stoichiometry of the reaction:

1 mole of calcium oxalate is produced from 1 mole of calcium ion

So, 0.0033 moles of calcium oxalate is produced from =
`(1)/(1)* 0.0033=0.0033mol` of calcium ions

• Now, calculating the mass of calcium ions by using equation 1, we get:

Moles of calcium ions = 0.0033 moles

Molar mass of calcium ions = 40 g/mol

Putting values in equation 1, we get:

`0.0033mol=\frac{\text{Mass of calcium ions}}{40g/mol}\\\\\text{Mass of calcium ions}=(0.0033mol* 40g/mol)=0.132g`

• To calculate the mass percentage of calcium ions in milk, we use the equation:

`\text{Mass percent of calcium ions}=\frac{\text{Mass of calcium ions}}{\text{Mass of milk}}* 100`

Mass of milk = 125 g

Mass of calcium ions = 0.132 g

Putting values in above equation, we get:

`\text{Mass percent of calcium ions}=(0.132g)/(125g)* 100=0.107\%`

Hence, the mass percent of calcium in milk is 0.107 %