# I was wondering if anyone could provide me with a method to solving as I do not know what I am doing wrong after multiple attempts?

A portland cement sample contained 20% SiO2 by weight derived from two silicate compounds, SiO2.2CaO and SiO2.3CaO that are present in the cement in the mole ratio 3 : 4. Determine the percent by weight of each silicate compound in the cement.

Answer key from book : 24.57%, 43.43%

Assuming the first silicate compound as W1 & the second one as W2 and their overall composition contribution to the sample as 100%. I got one equation W1+W2= 357.4 and using the mole ratios given, I equated them to get W1= .564 *W2. upon resubstituting that into the first equation, I got 128.8 g for W1 and 228.51 g for W2. Calculating the percentages for each, I got 36.03% and 63.9 % respectively. –

Note: I do find the percentages from the answer key weird too as they don't quite indicate a 100% composition contribution from the constituents. Or, maybe I am just missing something.

The help is much appreciated.

• You should include you solution as well.l, not just your result. Commented Feb 9, 2020 at 17:40
• Ok. Assuming the first silicate compound as W1 & the second one as W2 and their overall composition contribution to the sample as 100%. I got one equation W1+W2= 357.4 and using the mole ratios given, I equated them to get W1= .564 *W2. upon resubstituting that into the first equation, I got 128.8 g for W1 and 228.51 g for W2. Calculating the percentages for each, I got 36.03% and 63.9 % respectively. Commented Feb 9, 2020 at 17:46
• Technical note: The question improvement should be done in the question itself, not in the comments. Factual note: There is not said W1&W2 contribution is 100%. You also do not respect their ratio 3:4. Commented Feb 9, 2020 at 17:51
• Yes, I think this is the main mistake. w1+w2 is not equal to the weight of cement. Commented Feb 9, 2020 at 17:52
• SE is not just a forum, questions and answers should be written in a way so they can be found and used later by someone else, who gets stuck with a similar problem. Instead of trying to be ironic, you should rather do something about it.
– Karl
Commented Feb 9, 2020 at 22:28

This problem seems to have wrong data ! Look ! The molar mass of $$Ca_2SiO_4$$ is $$172$$. The molar mass of $$Ca_3SiO_5$$ is $$228$$. If you mix $$3$$ moles $$Ca_2SiO_4$$, (= $$516$$ g), and $$4$$ moles $$Ca_3SiO_5$$, (= $$912$$ g), the total weight is : $$516 + 912 = 1428$$ g. This mixture contains $$3+4 = 7$$ moles of $$SiO_2$$, which weighs $$420$$ g. In this mixture, the ratio $$SiO_2$$/total mass = $$420/1428$$ = $$29.4$$%. Well. This is contrary to the initial data saying that the proportion of $$SiO_2$$ is $$20$$% !!! Where is the mistake or the misconception ?