Question

A toothpaste contains 0.226 % by mass sodium fluoride used to prevent dental caries and 0.31 % by mass triclosan C12H7Cl3O2, an antigingivitis agent. One tube contains 119 g of toothpaste

A). How many moles of NaF are in the tube of toothpaste? Express your answer using three significant figures.

B). How many fluoride ions, F−, are in the tube of toothpaste? Express your answer using three significant figures.

C). How many grams of sodium ion, Na+, are in 2.30 g of toothpaste? Express your answer using three significant figures.

D). How many molecules of triclosan are in the tube of toothpaste? Express your answer using two significant figures.

Answer 1

A) 0.226% by mass sodium fluoride means for every 100 g of toothpaste we have 0.226 g of NaF in it.

So, 119g of toothpaste will contain 119*0.226/100 = 0.26894 g of NaF

The molar mass of NaF = molar mass of Na + molar mass of F = 23 + 19 = 42 g/mol

So, moles of NaF in 0.226 g = weight of NaF/molar mass of NaF = 0.26894/42 = 0.00640 moles Here we can neglect the values after 640 as we have already got the three significant figures 6,4 and 0. While calculating significant figures we need to neglect all the zeros before the first significant figure which in this case is 6

B) Each mole of NaF will contribute 1 mole of F- as NaF -> Na+ + F- so, no. of moles of F- = no. of moles of NaF in a toothpaste tube = 0.00640 moles

C) Each mole of NaF will contribute 1 mole of Na+ as NaF -> Na+ + F- so, no. of moles of Na+ = no. of moles of NaF in a toothpaste tube = 0.00640 moles

As 119 g of toothpaste has 0.00640 moles of NaF 2.5g of paste will contain 2.5*0.00640/119 moles of NaF = 1.345*10^-4 moles of NaF = 1.345*10^-4 moles of Na+

So, weight of Na+ in 1.345*10^-4 moles = 1.345*10^-4*23 (weight = moles*molecular mass)

So, weight = 0.00309 g Here we can neglect the values after 309 as we have already got the three significant figures 3,0 and 9

D) The toothpaste also contains 0.31% by mass triclosan C12H7Cl3O2.

So,0.31% by mass triclosan means for every 100 g of toothpaste we have 0.31g of triclosan in it.

so, 119g toothpaste will contain 119*0.31/100 g triclosan = 0.3689 g

Molecular mass of triclosan = 12*molar mass of C + 7*molar mass of H + 3*molar mass of Cl + 2*molar mass of O = 12*12 + 7*1 + 3*35.45 + 2*16 = 289.35

so, moles of triclosan in 119 g of toothpaste = weight of triclosan/molecular mass of triclosan = 0.3689/289.35 = 0.0012749 moles

Every mole of triclosan contains 6.022*10²³ molecules of triclosan so,

0.0012749 moles of triclosan will contatin 0.0012749 *6.022*10²³ molecules of triclosan = 7.667*10²⁰ To give answer with just two significant figures we must discard the 67 at the end and as there is value 6(value >5) after the value 7.6 we must round off the 6 to 7. Note 10²⁰ doesn't contribute to signifcant figures. So, only 7.7 are present as the two significant figures so, ans is 7.7*10²⁰