Answer:
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The heat cause 300g water temperature increase from 20 to 26 celcius. The heat transferred would be: 300g * (26 °C -20 °C) *4.2 joule/gram °C= 7560J
The unknown substance is added to the water, so its final temperature should be the same as the water. The calculation would be:
7560J= 124g * (100-26)* specific heat
specific heat= 7560J / 124g / 74 °C= 0.8238 J/gram °C
Oxidation state of Pb in PbO2 is +4.
Oxidation state of Pb in PbCl2 is +2.
Oxidation state of Pb in Pb2O is +1.
Oxidation state of Pb in Pb4O3 is +6/4.
Hence option A. PbO2 is correct.
Hope this helps, have a nice day!