Molarity HNO3 = 0.85 M
Volume HNO3 = 38.5 mL in liters: 38.5 / 1000 => 0.0385 L
Molarity Ba(OH)2 = ?
Volume Ba(OH)2 = 20.0 mL / 1000 => 0.02 L
Number of moles HNO3 :
n = M x V
n = 0.85 x 0.0385 => 0.032725 moles
mole ratio :
Ba(OH)2 + 2 HNO3 = Ba(NO3)2 + 2 H2O
1 mole Ba(OH)2 ----------- 2 moles HNO3
( moles Ba(OH)2 ) -------- 0.032725 HNO3
moles Ba(OH)2 = 0.032725 x 1 / 2
moles Ba(OH)2 = 0.032725 / 2
= 0.0163625 moles of Ba(OH)2
Molarity Ba(OH)2 :
M = n / V
M =0.0163625 / 0.02
= 0.82 M
hope this helps!